diff --git a/dirac.cfg b/dirac.cfg
index 0c255e2d89a..f0ebacdc7ba 100644
--- a/dirac.cfg
+++ b/dirac.cfg
@@ -1164,6 +1164,15 @@ Operations
       pilotVORepoBranch = master # Branch to use
       workDir = /tmp/pilot3Files # Local work directory on the masterCS for synchronisation
     }
+    # Installation specific help shown by dirac-proxy-init when a proxy can't be created
+    # Use "\n" for line breaks as values can't span multiple lines
+    ProxyInit
+    {
+      # Shown when the certificate DN is not registered in DIRAC
+      NotRegisteredMessage = New users must join the VO, see:\n    https://example.com/join-the-vo
+      # Shown when the VOMS server refuses to give the user an attribute certificate
+      VOMSFailureMessage = Check your certificate is linked to your account in IAM, see:\n    https://example.com/link-certificate
+    }
     Services
     {
       # See http://dirac.readthedocs.io/en/latest/AdministratorGuide/Resources/Catalog/index.html
diff --git a/setup.cfg b/setup.cfg
index b5eddc216bd..26cf4926c7a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -181,6 +181,7 @@ console_scripts =
     dirac-admin-sysadmin-cli = DIRAC.FrameworkSystem.scripts.dirac_admin_sysadmin_cli:main [admin]
     dirac-admin-update-instance = DIRAC.FrameworkSystem.scripts.dirac_admin_update_instance:main [admin]
     dirac-admin-update-pilot = DIRAC.FrameworkSystem.scripts.dirac_admin_update_pilot:main [admin]
+    dirac-admin-debug-user = DIRAC.FrameworkSystem.scripts.dirac_admin_debug_user:main
     dirac-admin-users-with-proxy = DIRAC.FrameworkSystem.scripts.dirac_admin_users_with_proxy:main [admin]
     dirac-install-component = DIRAC.FrameworkSystem.scripts.dirac_install_component:main [server]
     dirac-proxy-destroy = DIRAC.FrameworkSystem.scripts.dirac_proxy_destroy:main
diff --git a/src/DIRAC/FrameworkSystem/scripts/dirac_admin_debug_user.py b/src/DIRAC/FrameworkSystem/scripts/dirac_admin_debug_user.py
new file mode 100644
index 00000000000..7a3291feac8
--- /dev/null
+++ b/src/DIRAC/FrameworkSystem/scripts/dirac_admin_debug_user.py
@@ -0,0 +1,274 @@
+#!/usr/bin/env python
+"""Help to understand why a user is unable to get a proxy.
+
+The user can be given as a certificate DN (as printed by dirac-proxy-init or
+"openssl x509 -subject"), a DIRAC username, a CERN account, an email address or
+a CERN person ID. If there is no exact match, similar users from the registry
+are shown instead.
+
+As an unregistered user cannot query the configuration, this command is
+intended to be run by a colleague on behalf of the user having problems.
+
+Example:
+  $ dirac-admin-debug-user '/DC=ch/DC=cern/OU=Organic Units/OU=Users/CN=jdoe/CN=123456/CN=Jane Doe'
+  $ dirac-admin-debug-user --ca '/DC=ch/DC=cern/CN=CERN Grid Certification Authority' jdoe
+"""
+
+import re
+from datetime import date, datetime, timedelta, timezone
+from difflib import SequenceMatcher
+
+from DIRAC import gConfig, exit as dexit
+from DIRAC.Core.Base.Script import Script
+
+MIN_SCORE = 0.75
+# Scores at or above this mean the user is very likely the same person
+STRONG_SCORE = 0.95
+# Proxy CNs are appended as large random integers, much longer than CERN person IDs
+PROXY_CN_RE = re.compile(r"/CN=(\d{8,}|proxy|limited proxy)$")
+
+
+def normaliseDN(dn):
+    """Convert the various ways a DN can be written to the slash separated form used by DIRAC"""
+    dn = dn.strip()
+    if dn.startswith(("subject=", "issuer=")):
+        dn = dn.split("=", 1)[1].strip()
+    if not dn.startswith("/"):
+        # OpenSSL 3 style: "DC = ch, DC = cern, OU = Organic Units, CN = Jane Doe"
+        dn = "/" + "/".join(re.sub(r"\s*=\s*", "=", part.strip()) for part in dn.split(","))
+    stripped = False
+    while match := PROXY_CN_RE.search(dn):
+        dn = dn[: match.start()]
+        stripped = True
+    return dn, stripped
+
+
+def splitDN(dn):
+    """Split a slash separated DN into a list of (key, value) tuples"""
+    return [tuple(part.split("=", 1)) for part in dn.strip("/").split("/") if "=" in part]
+
+
+def nameFromDN(dn):
+    """Get the last non numeric CN which, for most CAs, is the person's name"""
+    names = [value for key, value in splitDN(dn) if key == "CN" and not value.isdigit()]
+    return names[-1].lower() if names else ""
+
+
+def loadUsers():
+    users = {}
+    result = gConfig.getSections("/Registry/Users")
+    if not result["OK"]:
+        raise RuntimeError(result["Message"])
+    for username in result["Value"]:
+        path = f"/Registry/Users/{username}"
+        users[username] = {
+            "DN": gConfig.getValue(f"{path}/DN", []),
+            "CA": gConfig.getValue(f"{path}/CA", []),
+            "Email": gConfig.getValue(f"{path}/Email", ""),
+            # "None" is used as a placeholder by the synchronisation
+            "Suspended": [vo for vo in gConfig.getValue(f"{path}/Suspended", []) if vo != "None"],
+            "PrimaryCERNAccount": gConfig.getValue(f"{path}/PrimaryCERNAccount", ""),
+            "CERNAccountType": gConfig.getValue(f"{path}/CERNAccountType", ""),
+            "CERNPersonId": gConfig.getValue(f"{path}/CERNPersonId", ""),
+            "AffiliationEnds": gConfig.getOptionsDict(f"{path}/AffiliationEnds").get("Value", {}),
+        }
+    return users
+
+
+def scoreDN(queryDN, info):
+    """Return (score, reason) for how likely it is that a user corresponds to the given DN"""
+    queryCNs = {value.lower() for key, value in splitDN(queryDN) if key == "CN"}
+    queryName = nameFromDN(queryDN)
+    best = (0.0, "")
+    for dn in info["DN"]:
+        if dn == queryDN:
+            return 1.0, "identical DN"
+        # Only compare the names as the rest of the DN is often identical for everyone with the same CA
+        best = max(best, (SequenceMatcher(None, queryName, nameFromDN(dn)).ratio(), "similar name"))
+    # A matching CERN person ID or account name is a much stronger signal than the name
+    if info["CERNPersonId"] and info["CERNPersonId"] in queryCNs:
+        best = max(best, (0.98, f"same CERN person ID ({info['CERNPersonId']})"))
+    if info["PrimaryCERNAccount"] and info["PrimaryCERNAccount"].lower() in queryCNs:
+        best = max(best, (0.95, f"same CERN account ({info['PrimaryCERNAccount']})"))
+    return best
+
+
+def findUsers(query, users, limit):
+    """Find the users matching a query, returns (exact, similar, normalisedDN)"""
+    if query.startswith("/") or "=" in query:
+        dn, stripped = normaliseDN(query)
+        if stripped:
+            print(f"NOTE: Removed proxy CN(s) from the DN, using {dn}\n")
+        scored = sorted(
+            ((*scoreDN(dn, info), username) for username, info in users.items()),
+            reverse=True,
+        )
+        exact = [username for score, _, username in scored if score == 1.0]
+        similar = [(username, score, reason) for score, reason, username in scored if MIN_SCORE <= score < 1.0]
+        return exact, similar[:limit], dn
+
+    q = query.strip().lower()
+    exact = [
+        username
+        for username, info in users.items()
+        if q in (username.lower(), info["Email"].lower(), info["PrimaryCERNAccount"].lower(), info["CERNPersonId"])
+    ]
+    similar = []
+    if not exact:
+        scored = sorted(
+            (
+                (
+                    max(SequenceMatcher(None, q, s.lower()).ratio() for s in (username, info["Email"].split("@")[0])),
+                    username,
+                )
+                for username, info in users.items()
+            ),
+            reverse=True,
+        )
+        similar = [(username, score, "similar username/email") for score, username in scored if score >= MIN_SCORE]
+    return exact, similar[:limit], None
+
+
+def canReadProxyDB():
+    from DIRAC.Core.Security import Properties
+    from DIRAC.Core.Security.ProxyInfo import getProxyInfo
+
+    result = getProxyInfo()
+    if not result["OK"]:
+        return False
+    return Properties.PROXY_MANAGEMENT in result["Value"].get("groupProperties", [])
+
+
+def getUploadedProxies(username):
+    from DIRAC.FrameworkSystem.Client.ProxyManagerClient import gProxyManager
+
+    result = gProxyManager.getDBContents({"UserName": [username]})
+    if not result["OK"]:
+        return result
+    names = result["Value"]["ParameterNames"]
+    return {"OK": True, "Value": [dict(zip(names, record)) for record in result["Value"]["Records"]]}
+
+
+def checkUser(username, info, queryDN, queryCA, checkProxies):
+    """Print the details of a user and return a list of problems found"""
+    from DIRAC.ConfigurationSystem.Client.Helpers import Registry
+
+    problems = []
+    groups = Registry.getGroupsForUser(username).get("Value", [])
+
+    print(f"* {username}")
+    for dn in info["DN"]:
+        print(f"    DN            : {dn}{'  <-- matches' if dn == queryDN else ''}")
+    for ca in info["CA"]:
+        print(f"    CA            : {ca}")
+    print(f"    Email         : {info['Email'] or '-'}")
+    if info["PrimaryCERNAccount"]:
+        print(f"    CERN account  : {info['PrimaryCERNAccount']} ({info['CERNAccountType'] or 'unknown type'})")
+    if info["CERNPersonId"]:
+        print(f"    CERN person ID: {info['CERNPersonId']}")
+    print(f"    Groups        : {', '.join(groups) or '-'}")
+
+    if queryCA and queryCA not in info["CA"]:
+        problems.append(f"the given CA is not registered for this user (registered: {', '.join(info['CA']) or '-'})")
+    if not groups:
+        problems.append("the user is not a member of any group")
+    if info["Suspended"]:
+        problems.append(f"the user is suspended in: {', '.join(info['Suspended'])}")
+
+    today = date.today()
+    for vo, endDate in info["AffiliationEnds"].items():
+        print(f"    Affiliation   : {vo} until {endDate}")
+        try:
+            endDate = date.fromisoformat(endDate)
+        except ValueError:
+            continue
+        if endDate < today:
+            problems.append(f"the {vo} affiliation ended on {endDate}")
+        elif endDate < today + timedelta(days=30):
+            problems.append(f"the {vo} affiliation ends soon ({endDate})")
+
+    if checkProxies:
+        result = getUploadedProxies(username)
+        if not result["OK"]:
+            print(f"    Proxies       : failed to query ProxyManager: {result['Message']}")
+        elif not result["Value"]:
+            print("    Proxies       : none uploaded")
+            problems.append("no proxy has been uploaded to the ProxyManager")
+        else:
+            now = datetime.now(timezone.utc).replace(tzinfo=None)
+            validProxies = []
+            for proxy in result["Value"]:
+                expiry = proxy["ExpirationTime"]
+                state = "valid" if expiry > now else "EXPIRED"
+                print(f"    Proxy         : {proxy['UserDN']} until {expiry:%Y-%m-%d %H:%M} ({state})")
+                if expiry > now and proxy["UserDN"] in info["DN"]:
+                    validProxies.append(proxy)
+            if not validProxies:
+                problems.append("there is no valid uploaded proxy for any of the registered DNs")
+
+    for problem in problems:
+        print(f"    PROBLEM       : {problem}")
+    if not problems:
+        print("    No problems found in the DIRAC registry")
+    print()
+    return problems
+
+
+@Script()
+def main():
+    Script.registerSwitch("", "ca=", "CA (issuer DN) of the user's certificate")
+    Script.registerSwitch("n:", "limit=", "Maximum number of similar users to show (default 5)")
+    Script.registerArgument(
+        "user: certificate DN, DIRAC username, CERN account, email or CERN person ID", mandatory=True
+    )
+    Script.parseCommandLine(ignoreErrors=False)
+
+    queryCA = None
+    limit = 5
+    for switch, value in Script.getUnprocessedSwitches():
+        if switch == "ca":
+            queryCA = normaliseDN(value)[0]
+        elif switch in ("n", "limit"):
+            limit = int(value)
+    query = " ".join(Script.getPositionalArgs())
+
+    users = loadUsers()
+    exact, similar, queryDN = findUsers(query, users, limit)
+
+    checkProxies = canReadProxyDB()
+    if not checkProxies:
+        print("NOTE: Uploaded proxies will not be checked as this requires the ProxyManagement property\n")
+
+    problems = []
+    if exact:
+        print(f"Found {len(exact)} user(s) matching {query!r}:\n")
+        for username in exact:
+            problems += checkUser(username, users[username], queryDN, queryCA, checkProxies)
+    else:
+        problems.append("no exact match")
+        if similar:
+            print(f"No user exactly matches {query!r}, the most similar users are:\n")
+            for username, score, reason in similar:
+                print(f"  (score {score:.2f}: {reason})")
+                if queryDN and score >= STRONG_SCORE:
+                    print("  This is very likely the same person: the new certificate must be registered with the VO")
+                checkUser(username, users[username], queryDN, queryCA, checkProxies)
+        else:
+            print(f"No user exactly matches {query!r} and no similar users were found.\n")
+
+    if problems or not exact:
+        print(
+            "If the DN has changed (e.g. renewed certificate, name change or a new CA) the new\n"
+            "certificate must be registered with the VO (e.g. linked to the user's account in IAM).\n"
+            "Changes in the VO membership service can take a few hours to propagate to DIRAC."
+        )
+        dexit(1)
+    print(
+        "The user looks fine in the DIRAC registry. If VOMS still refuses to give an attribute certificate,\n"
+        "check the certificate subject and issuer registered with the VO (e.g. in IAM)."
+    )
+    dexit(0)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/src/DIRAC/FrameworkSystem/scripts/dirac_proxy_init.py b/src/DIRAC/FrameworkSystem/scripts/dirac_proxy_init.py
index 5da65c97e8d..cc1df147cdc 100755
--- a/src/DIRAC/FrameworkSystem/scripts/dirac_proxy_init.py
+++ b/src/DIRAC/FrameworkSystem/scripts/dirac_proxy_init.py
@@ -9,12 +9,14 @@
 import datetime
 import glob
 import os
+import shlex
 import sys
 import time
 
 import DIRAC
 from DIRAC import S_ERROR, S_OK, gLogger
 from DIRAC.ConfigurationSystem.Client.Helpers import Registry
+from DIRAC.ConfigurationSystem.Client.Helpers.Operations import Operations
 from DIRAC.Core.Base.Script import Script
 from DIRAC.Core.Security import VOMS, ProxyInfo, X509Chain
 from DIRAC.Core.Security.DiracX import addTokenToPEM
@@ -50,6 +52,8 @@ def __init__(self, piParams):
         self.__issuerCert = False
         self.__proxyGenerated = False
         self.__uploadedInfo = {}
+        # Name of the ProxyInit/<option> help message to show if the proxy can't be created
+        self.helpNeeded = None
 
     def getIssuerCert(self):
         if self.__issuerCert:
@@ -117,6 +121,8 @@ def createProxy(self):
         resultProxyGenerated = ProxyGeneration.generateProxy(piParams)
         if not resultProxyGenerated["OK"]:
             gLogger.error(resultProxyGenerated["Message"])
+            if "is not registered" in resultProxyGenerated["Message"]:
+                self.printHelp("NotRegisteredMessage")
             sys.exit(1)
         self.__proxyGenerated = resultProxyGenerated["Value"]
         return resultProxyGenerated
@@ -169,6 +175,31 @@ def printInfo(self):
             for userDN in self.__uploadedInfo:
                 gLogger.notice(f" {userDN.ljust(maxDNLen)} | {self.__uploadedInfo[userDN].strftime('%Y/%m/%d %H:%M')}")
 
+    def printHelp(self, option):
+        """Print information to help the user understand why they can't get a proxy
+
+        This includes the certificate details needed for someone else to debug the
+        registration and the installation specific message from Operations/ProxyInit/<option>
+        """
+        chain = X509Chain.X509Chain()
+        if self.__piParams.certLoc and chain.loadChainFromFile(self.__piParams.certLoc)["OK"]:
+            # Same certificate as used by ProxyGeneration to find the user DN
+            cert = chain.getCertInChain(-1)["Value"]
+            subject = cert.getSubjectDN().get("Value")
+            issuer = cert.getIssuerDN().get("Value")
+            if subject and issuer:
+                gLogger.notice(
+                    f"\nYour certificate subject (DN) is:\n    {subject}\n"
+                    f"and its issuer (CA) is:\n    {issuer}\n"
+                    "If you need help, someone with a working proxy can check your registration by running:\n"
+                    f"    dirac-admin-debug-user --ca {shlex.quote(issuer)} {shlex.quote(subject)}"
+                )
+
+        ops = Operations(group=self.__piParams.diracGroup) if self.__piParams.diracGroup else Operations()
+        # CFG values can't span multiple lines so allow "\n" to be used instead
+        if message := ops.getValue(f"ProxyInit/{option}", "").replace("\\n", "\n"):
+            gLogger.notice(f"\n{message}")
+
     def checkCAs(self):
         caDir = getCAsLocation()
         if not caDir:
@@ -215,13 +246,19 @@ def doTheMagic(self):
             return result
 
         if not resultProxyWithVOMS["OK"]:
-            if "returning a valid AC for the user" in resultProxyWithVOMS["Message"]:
-                gLogger.error(resultProxyWithVOMS["Message"])
-                gLogger.error("\n Are you sure you are properly registered in the VO?")
-            elif "Missing voms-proxy" in resultProxyWithVOMS["Message"]:
+            message = resultProxyWithVOMS["Message"]
+            if "Missing voms-proxy" in message:
                 gLogger.notice("Failed to add VOMS extension: no standard grid interface available")
             else:
-                gLogger.error(resultProxyWithVOMS["Message"])
+                if "returning a valid AC for the user" in message:
+                    message += "\n\nAre you sure you are properly registered in the VO?"
+                    self.helpNeeded = "VOMSFailureMessage"
+                if self.__piParams.strict:
+                    # Printed by main, avoid showing the error twice
+                    return S_ERROR(message)
+                gLogger.error(message)
+                if self.helpNeeded:
+                    self.printHelp(self.helpNeeded)
             if self.__piParams.strict:
                 return resultProxyWithVOMS
 
@@ -250,6 +287,8 @@ def main():
     resultDoTheMagic = pI.doTheMagic()
     if not resultDoTheMagic["OK"]:
         gLogger.fatal(resultDoTheMagic["Message"])
+        if pI.helpNeeded:
+            pI.printHelp(pI.helpNeeded)
         sys.exit(1)
 
     pI.printInfo()