We found a much easier solutions to solve the problem, without re-provisioning the server.
Since the listener complained about errors in /var/lib/imap/private/idmap.ldb I opened the file with vi, to check the content:
ldbedit -e vi -H /var/lib/samba/private/idmap.ldb
I didn’t not find any errors but nevertheless decided to save the file and not just leave it. This obviously saved the file in an error free state and listener now is able to read the file again.
Thanks for your help,
Stefan