Login fails

[SolarSimon]

Hello everyone,
I have had discourse installed for a long time. (lately 3.1.3)
Recently a user informed me that the login no longer works.

Currently nothing happens during login, the login window remains and shows "Signing In" forever.
If I move the Discourse app to another subdomain, everything seems to work. If I put it back on the original subdomain, the login does not work again.
Even a fresh installation shows the same error.

What I have already tried.

• I quickly found out that a plugin could be the problem. I removed all subfolders from the plugins folder. At that point, the login worked again. I put the plugins back in one by one. Suddenly the login no longer worked at all.
• I have restored backups. First the login worked again without plugins. So I tried again to find out which plugin was causing problems.
• Now there is no login at all, with or without plugins. Not even with a fresh installation.
• On another subdomain the login works, even with the plugins.

How can the domain cause the error? What should I do?

[nebulon]

Did you narrow down which plugin is causing this? Are there any errors shown on the server side? Also just to rule out the obvious, have you tried with another browser, which may not have cached something for that domain?

[SolarSimon]

Okay, I've come one step further.

The situation with the domain was apparently caused by Firefox. With Vivaldi there is a successful login again, or an error, depending on the plugins. How unnecessary

Actually Discourse only works with one plugin, but the plugin itself does not work: Discourse User Notes

With the other tested plugins nothing works, only the error message:

"Oops
The software powering this discussion forum encountered an unexpected problem...."
I only tested two further Plugins: check-list and reactions

My test procedure is simply to remove the plugin folder and restart the app. I have no idea whether this is a reliable test.