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.