After Update some Users cannot log in any more, others still can

Nextcloud version (eg, 20.0.5): 23.0.5
Operating system and version (eg, Ubuntu 20.04): unknown
Apache or nginx version (eg, Apache 2.4.25): unknown
PHP version (eg, 7.4): 7.3.10

As a non-profit organization we use Nextcloud for several years. Now, our provider did an update from version 19 to 23.0.5. After that some users cannot log in any more. After pressing the “log in” button, nothing visible happens for a long time. After that the following error message occurs:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at ... to inform them of the time this error occurred, and the actions you performed just before this error.
Mor information about this error may be available in the server error log.

The support of the provider insists that this is due to a local problem of the user’s computer. So they say, they cannot help.

I tried different users with the same computer and the same browser, and for some users the login is successful, for others not. If I enter a false password, an error message is displayed as expected. So I guess it is no password problem. And I guess it is a server problem.

There are no messages in the log.

I do not have access to the server’s console, and I cannot execute Linux commands.

If I open the console on the browser while loading the page, there are sometimes error messages like:

Cannot load due to access control checks.

If I enter those links in the browser line, the rescource can be loaded.

This lasts for 3 weeks now, and the provider does not see it as their problem.

I hope I could get some hints or suggestions what to do or what to say to the provider.

You could ask your provider to fill out this support template. Sounds like your provider is not good and you should seriously consider finding someone else. As you’ve already discovered, resolving any technical issues will be entirely their problem.

You are missing the required support template. Please fill this form out and edit into your post.
This will give us the technical info and logs needed to help.

Try asking your users to clear their browser cache and see if that helps them be able to log in.

Clearing the browser cache was indeed the first attempt to solve the problem, but unfortunately, it did not help.

The provider does not want to send the logs. But they said they met a Nextcloud developer to discuss the problem. Regarding the result of that meeting they wrote: “wir haben alle details vorgelegt, logs usw. Auch der nextcloud developer selbst geht von einem 3rd party app problem aus da auch keine webserver errors vorhanden sind, impersonate funktioniert und ja zahlreiche 3rd party apps von ihnen genutzt wurden und somit bei dem user der sich nicht anmelden kann, irgendwo einträge vorhanden sind die den login verhindern.” That is in English: “we have submitted all details, logs, etc… Also the nextcloud developer himself assumes a 3rd party app problem because there are no webserver errors, impersonate works and numerous 3rd party apps were used by them and therefore the user who can not log in, somewhere entries are present that prevent the login.”

First: We did not use “numerous 3rd party apps”. We used files, calendar, contacts, group folders, access rights. We did not install experimental apps.

I understand from that statement that I did something wrong, I did not know, and now they all - the provider and the Nextcloud developer - are free and need not help any more.

I would guess that a developer should be able to find the point of crash within a certain PHP script and thus find the database entry that causes the crash if any. (And we provided user name and password for them to check.)

And I would guess that that meeting was to be paid by the provider. So what quality has the paid Nextcloud service?

As you can imagine I am frustrated, and I do not know what to do now.