[Bug]: Error on accounts page, when manager of accounts has been deleted

⚠️ 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

When opening the accounts page (/settings/users) or specifically the "Disabled accounts" view (/settings/users/disabled), the following error message is shown:

An error occurred during the request. Unable to proceed.

In the network tab of the browser developer tools, the following GET request /ocs/v2.php/cloud/users/example-manager has this response:

{
    "ocs": {
        "meta": {
            "status": "failure",
            "statuscode": 404,
            "message": "User does not exist"
        },
        "data": []
    }
}

The "example-manager" account really does not exist on our Nextcloud instance.
However, it is still referenced as the manager on another account. For this reason the accounts page frontend still tries to load it (/ocs/v2.php/cloud/users/details?offset=0&limit=25&search=):

{
    "ocs": {
        "meta": {
            "status": "ok",
            "statuscode": 200,
            "message": "OK"
        },
        "data": {
            "users": {
                "example-user": {
                    "enabled": true,
                    "storageLocation": "\/var\/www\/html\/data\/example-user",
                    "id": "example-user",
                    "firstLoginTimestamp": 0,
                    "lastLoginTimestamp": 1706374106,
                    "lastLogin": 1706374106000,
                    "backend": "Database",
                    "subadmin": [],
                    "quota": {
                        "free": 524288000,
                        "used": 0,
                        "total": 524288000,
                        "relative": 0,
                        "quota": 524288000
                    },
                    "manager": "example-manager",
                    "email": "example-user",
                    "additional_mail": [],
                    "displayname": "example-user",
                    "display-name": "example-user",
                    "phone": "",
                    "address": "",
                    "website": "",
                    "twitter": "",
                    "bluesky": "",
                    "fediverse": "",
                    "organisation": "",
                    "role": "",
                    "headline": "",
                    "biography": "",
                    "profile_enabled": "1",
                    "pronouns": "",
                    "groups": [
                        "example-group"
                    ],
                    "language": "en",
                    "locale": "",
                    "timezone": "Europe\/Berlin",
                    "notify_email": null,
                    "backendCapabilities": {
                        "setDisplayName": true,
                        "setPassword": true
                    }
                },
            }
        }
    }
}

Steps to reproduce

1. Assign a manger to an account
2. Delete the manger account
3. Open the accounts page

Expected behavior

No error message is shown.
Either by gracefully handling this or by removing the manager from the account on the backend.

Nextcloud Server version

32

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Apache (supported)

Database engine version

MariaDB

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

Upgraded to a MAJOR version (ex. 31 to 32)

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

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start by reproducing the failure at /settings/users and /settings/users/disabled, then inspect the requests to /ocs/v2.php/cloud/users/details and /ocs/v2.php/cloud/users/example-manager. Trace how an account referencing a deleted manager is handled, and verify that the accounts page loads without an error, either by gracefully handling the missing manager or removing the stale reference.