Thanks, found it … but …
At first I disabled the maintenance mode in the config.php:
'maintenance' => false
After that the screen appears with an update button, so far so good. After pressing it, I got a warning of incompatible or missing apps. In my case the missing apps are the cause, I don’t find any of them in my apps folder.
Than I found your hint to disable the apps using a database tool (Upgrade impossoble). And now it seems I got a real problem. For whatever reason I set up my owncloud installation using a PostgreSQL database. Unfortunately my hosting provider doesn’t give me any tools to adminster this kind of database (a side note: If I had choosen MySQL I could use PHPMAyAdmin 
)
So I’m afraid I have to set up my owncloud installation from scratch now … 
Or is there any other way out of this …