Hi Moritz,
thank you for the hint. Finally I got my owncloud updated and running again.
It was a little bit different than you described so I post my way of solving it. Perhaps someone may find it helpful:
1.) I could not run univention-app shell docker:
root@server1:~# univention-app shell docker
usage: univention-app shell [-h] [-i] [-t] app ...
univention-app shell: error: Unable to find app docker. Maybe "/usr/bin/univention-app update" to get the latest list of applications?
I tried univention-app shell owncloud and that gave me a shell inside owncloud/docker.
2.) Because in the docker container I had no sudo command, I logged in as www-data with su www-data.
3.) There I changed into /var/www/owncloud . From there I could run ./occ upgrade and afterwards ./occ maintenance:mode --off.
4.) I logged into owncloud as super-user “owncloud” and got a integrity error for the file resources/config/ca-bundle.crt. I don’t know what to do with it. Seems to me it’s related to the certificate of the server( I am using Let's encrypt, perhaps that’s changing the file ???). Anyhow the server is working again.
Any hint in how to solve the integrity error would be appreciated.