Hi Luca, thank you for the hint. Unfortunately 2 minutes to late.
In the meantime I had completely deleted Webtop, including the database.
Since the documentation does not describe how to uninstall it, I based myself on this description:
#----removing webtop databases and its user----
su - postgres
dropdb webtop
dropdb webtop5
dropuser sonicle
exit
After reinstalling Webtop, I discovered the user management within Webtop at first sight; not before.
If I really missed it or if the previous installation had not been done correctly… I don’t know.
The former error is eliminated now.
Thank you very much.