In the meanwhile I was able to identify the root cause… a long time ago I installed asterisk4ucs for testing purposes but uninstalled it again.
This uninstall was not complete as there were lots of LDAP Attributes / Objectclasses regarding to asterisk.
In the end it was indeed neccessary to follow this article:
So I made a ldap backup, removed/edited every entry with some sort of “astersik” in it, restored this modified one and the error message was gone.
I’m wondering why the uninstall of asterisk left over this amount of LDAP modifications…