Hello @Nikl4s,
has your UCS system been setup with the same fully qualified domain name (FQDN) that you are using to access ownCloud?
There are setups that use a different fqdn for external and internal access. There are already some postings at Univention Help. Maybe it helps to have a look at https://help.univention.com/tags/owncloud.
Best regards,
Nico