[Bug]: InvalidArgumentException: userId cannot be an empty string

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Exception logged: "userId cannot be an empty string"

Steps to reproduce

1. reopen yesterdays browser tabs while authenticated from proxy via REMOTE_USER
2. observe logs

Expected behavior

Auth issues should always be handled robustly. Any request should be handled as a request from an authorized user, or not. Never in some internal error state in between.

Nextcloud Server version

31

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.1

Web server

Nginx

Database engine version

PostgreSQL

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "version": "31.0.7.1",
        "config_is_read_only": true,
        "connectivity_check_domains": [],
        "check_for_working_wellknown_setup": false,
        "secret": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "maintenance_window_start": 21,
        "debug": false,
        "default_phone_region": "DE",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "updatechecker": false,
        "upgrade.disable-web": true,
        "updater.server.url": "***REMOVED SENSITIVE VALUE***",
        "updater.release.channel": "stable",
        "skeletondirectory": "",
        "default_language": "de",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "htaccess.RewriteBase": "\/",
        "allow_local_remote_servers": true,
        "loglevel": 2,
        "log_type": "syslog",
        "syslog_tag": "nextcloud",
        "logfile": "",
        "logdateformat": "Y-m-d H:i:s",
        "logtimezone": "UTC",
        "log.condition": {
            "apps": [
                "admin_audit"
            ]
        },
        "log_type_audit": "syslog",
        "syslog_tag_audit": "nextcloud",
        "logfile_audit": "",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "memcache.distributed": "\\OC\\Memcache\\Memcached",
        "memcache.locking": "\\OC\\Memcache\\Memcached",
        "memcached_servers": [
            [
                "\/run\/nextcloud\/memcached.sock",
                0
            ]
        ],
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "maintenance": false,
        "knowledgebaseenabled": false,
        "enable_previews": true,
        "allow_user_to_change_display_name": false,
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpauthtype": "LOGIN",
        "mail_smtpsecure": "ssl",
        "mail_smtpauth": 1,
        "trashbin_retention_obligation": "14, 60",
        "mail_send_plaintext_only": true,
        "theme": "",
        "has_internet_connection": false
    }
}

List of activated Apps

Enabled:
  - activity: 4.0.0
  - admin_audit: 1.21.0
  - app_api: 5.0.2
  - calendar: 5.3.7
  - circles: 31.0.0
  - cloud_federation_api: 1.14.0
  - comments: 1.21.0
  - contactsinteraction: 1.12.0
  - dav: 1.33.0
  - federatedfilesharing: 1.21.0
  - files: 2.3.1
  - files_accesscontrol: 2.0.0
  - files_automatedtagging: 2.0.0
  - files_downloadlimit: 4.0.0
  - files_reminders: 1.4.0
  - files_retention: 2.0.1
  - files_sharing: 1.23.1
  - files_trashbin: 1.21.0
  - files_versions: 1.24.0
  - logreader: 4.0.0
  - lookup_server_connector: 1.19.0
  - notifications: 4.0.0
  - oauth2: 1.19.1
  - privacy: 3.0.0
  - profile: 1.0.0
  - provisioning_api: 1.21.0
  - related_resources: 2.0.0
  - richdocuments: 8.7.3
  - serverinfo: 3.0.0
  - settings: 1.14.0
  - systemtags: 1.21.1
  - tasks: 0.16.1
  - text: 5.0.0
  - theming: 2.6.1
  - twofactor_backupcodes: 1.20.0
  - user_saml: 6.6.0
  - viewer: 4.0.0
  - webhook_listeners: 1.2.0
  - workflowengine: 2.13.0
Disabled:
  - bruteforcesettings: 4.0.0 (installed 1.0.3)
  - dashboard: 7.11.0 (installed 7.0.0)
  - encryption: 2.19.0
  - federation: 1.21.0 (installed 1.2.0)
  - files_external: 1.23.0
  - files_pdfviewer: 4.0.0 (installed 2.10.0)
  - nextcloud_announcements: 3.0.0 (installed 1.1)
  - password_policy: 3.0.0 (installed 1.3.0)
  - photos: 4.0.0 (installed 1.0.0)
  - recommendations: 4.0.0 (installed 0.8.0)
  - support: 3.0.0 (installed 1.1.0)
  - suspicious_login: 9.0.1
  - twofactor_nextcloud_notification: 5.0.0
  - twofactor_totp: 13.0.0-dev.0
  - updatenotification: 1.21.0 (installed 1.2.0)
  - user_ldap: 1.22.0
  - user_status: 1.11.0 (installed 1.8.1)
  - weather_status: 1.11.0 (installed 1.7.0)

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

{"reqId":"***redacted***","level":3,"time":"2025-07-31 06:39:29","remoteAddr":"***redacted***","user":"--","app":"index","method":"GET","url":"/apps/files/?dir=/redacted&fileid=1337","message":"{\"Exception\":\"InvalidArgumentException\",\"Message\":\"userId cannot be an empty string\",\"Code\":0,\"Trace\":[{\"file\":\"/nc/lib/private/Config/UserConfig.php\",\"line\":152,\"function\":\"assertParams\",\"class\":\"OC\\\\Config\\\\UserConfig\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/AllConfig.php\",\"line\":282,\"function\":\"hasKey\",\"class\":\"OC\\\\Config\\\\UserConfig\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/TemplateLayout.php\",\"line\":130,\"function\":\"getUserValue\",\"class\":\"OC\\\\AllConfig\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/legacy/OC_Template.php\",\"line\":120,\"function\":\"__construct\",\"class\":\"OC\\\\TemplateLayout\",\"type\":\"->\"},{\"file\":\"/nc/lib/public/AppFramework/Http/TemplateResponse.php\",\"line\":189,\"function\":\"fetchPage\",\"class\":\"OC_Template\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/AppFramework/Http/Dispatcher.php\",\"line\":159,\"function\":\"render\",\"class\":\"OCP\\\\AppFramework\\\\Http\\\\TemplateResponse\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/AppFramework/App.php\",\"line\":161,\"function\":\"dispatch\",\"class\":\"OC\\\\AppFramework\\\\Http\\\\Dispatcher\",\"type\":\"->\"},{\"file\":\"/nc/lib/private/Route/Router.php\",\"line\":315,\"function\":\"main\",\"class\":\"OC\\\\AppFramework\\\\App\",\"type\":\"::\"},{\"file\":\"/nc/lib/base.php\",\"line\":1040,\"function\":\"match\",\"class\":\"OC\\\\Route\\\\Router\",\"type\":\"->\"},{\"file\":\"/nc/index.php\",\"line\":24,\"function\":\"handleRequest\",\"class\":\"OC\",\"type\":\"::\"}],\"File\":\"/nc/lib/private/Config/UserConfig.php\",\"Line\":1668,\"message\":\"userId cannot be an empty string\",\"exception\":{},\"CustomMessage\":\"userId cannot be an empty string\"}","userAgent":"***redacted***","version":"31.0.7.1"}

Additional info

I do not yet know ways to immediately or reliably trigger this. I have so far only seen it with with memcached enabled, possibly some non-atomic cache operation involving multiple keys?

Start with lib/private/Config/UserConfig.php around lines 152 and 1668, then trace the request through AllConfig.php and TemplateLayout.php as shown in the log. Reproduce the browser-tab request with proxy REMOTE_USER and SAML authentication while memcached is enabled. Done means the request no longer reaches an internal state that passes an empty userId to UserConfig, with the authorization outcome handled consistently.