the Problem has been solved. The key in this case was the inclusion of the nesting Filter to get nested Groups working.
The format looks like this:
(&(sAMAccountName=#{username})(memberof:1.2.840.113556.1.4.1941:=CN=#{groupName},DC=your domain (objectCategory=person)(objectClass=user))
We included the category and class filter to rule out false positives like the nested security groups.