Yes I tried this but not much useful information.
In the end, I received a message saying the DB version too high for the Duplicati version I was using (I had tried to downgrade to an earlier version).
Uninstalling and re-installing did not help, not even after a manual clean of system files and registry.
Fortunately I had stored an old version of Duplicati on a separate disk, and that one started like a charm. So I did a brute file copy to the standard location of Duplicati, and from then on Duplicati started again without issues. I had to recreate my backup job though, because the old one got corrupted and there was no way to retrieve it or to restore the DB.
So it remains a mystery what went wrong and why.
At least, I have a running backup system again.