Note: This issue might only be related to hostpoint.ch installations.
Until the fix #508 it was not possible at all to use the updater on hostpoint.ch installations. After the mentioned fix, the update successes until step 11:

But that step (disabling the maintenance mode) leads to an error 500:
Internal Server Error
The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the webserver log.