Something wrong with web UI

Yes, exactly what I meant. You can see that the error is 502, meaning that your browser has a connecting to the proxy machine, but fails from the proxy server back to Duplicati for some reason.

Looking back at the original message you posted:

This clearly indicates that Duplicati has restarted for some reason and would explain the 502 errors and the UI experience.

Next question is then: why does it restart? The logs here do not help in that regard.
If Duplicati itself is crashing, there should be a crashlog-Duplicati.txt file in the config folder (since it has started the server), otherwise in the system temp folder (which normally is wiped on Docker restart).

Could it be some monitoring or similar that detects an out-of-memory or something similar that stops it?

This indicates that the backup starts (so the scheduler thinks it has started) but then crashes afterwards.

That is a different question, but it depends on how much data is included in the backup (number of files, number of versions, size of data, number of remote files, etc).

You can see from the journal file, that an operation is active on the database (or it crashed while it was writing).

Is the job that does not run using the Storj backend? There is another report that it crashes each time it tries to run a Storj backup.