I tried deleting and recreating site and shell-user with a new name, it works. Thank you.
However I can not reuse the old names, I think there is a small bug.
Deleting a shell-user in ISPConfig 3 this is not actually deleted.
Thus, creating a new shell-user in ISPConfig 3, re-using the old name does not change the ID.
web1 id 5009
shelluserweb1 id 5008
How do I actually delete a shell-user no longer used in ISPConfig 3 without making a mess?