Your UCS server tells your current version is out of maintenance and you should update to a newer version. But there are no newer versions shown to upgrade to.
Running univention-upgrade on the console does not show any available updates, either.
There might be no online repositories configured to use. Enable online repositories by:
echo > /etc/apt/sources.list apt update ucr set repository/online=true ucr commit /etc/apt/sources.list.d/* apt update
Afterwards, update should find new versions and you should be able to update.