First open /manager/includes/accesscontrol.inc.php
Go to line 66 and insert the following:
$manager_theme = ’MODxLight/’;
(Don’t forget the slash at the end)
Load the file up to your webserver.
Now you can login into the manager.
The design is completely broken, but you will be able to save the configuration and after the reload everything will look fine.
You should remove line 66 now, because you can’t change the manager theme otherwise.
The problem vanished
Thanks to onesmarthost for the help to bring me to this solution.