Since three days I get the following screen when I try to modify the user account. 

The server has not been upgraded, patched or otherwise changed since the error started. Everything else seems to work as normal.

Tried rebooting the server, restarting the service, checked the accounts used for this. Everything seems to be fine... The forum does not show any topics similar. AD Audit runs on the same server and is also running fine. It seems to happen only when modifying an existing account. 

Any ideas?