It is not possible to login to Owncloud any more. Owncloud complains about having lost the connection to LDAP.
First, check DNS settings:
Then check if DNS settings work inside the Docker container as well:
univention-app shell owncloud ping <NAME-OF-YOUR-UCS>
If one of the above (or both) commands fail check you DNS configuration. In case the server is not found, simply add it to your DNS server in use.
Afterwards Owncloud can resolve the name and find the LDAP-base.