I solved the problem by editing /kernel/classes/datatypes/ezuser/ezldapuser.php A programmer of us had done modifications to that file, which were overwritten by the update files. Without this modifications ldap doesn't work here.
As it is not possible to attach a file here, I don't know how to let the community participate - maybe the modifications to ezldapuser.php fixes a bug? Anyway, if your interested just let me know and I send you the file.