Hello Christian,
I read your thread from March 2015, about the problems deleting the domain.
https://lists.fedoraproject.org/pipermail/389-users/2015-March/017880.html
I had the same problem, and finally I found a solution.
Somehow, this leaf object is not deleted when you remove the domain:
cn=kolab-admin,dc=dertest2,dc=de:
{"cn":"kolab-admin","objectclass":["top","ldapsubentry","nsroledefinition","nssimpleroledefinition","nsmanagedroledefinition"]}
The object class ldapsubentry is something special, I guess.
When I specifically delete this leaf first, then I can delete the
domain without any problems.
See more details and the fix here:
https://issues.kolab.org/show_bug.cgi?id=5100
Hope this helps,
Timotheus