There is no /etc/apt/sources.list.d/omvextras.list on the system.
But I created one based on your example and then rebooted the system.
Unfortunately, without success.
Amazingly, the system still provides all SMB shares on the LAN, even though it was so messed up by the failed upgrade.
Since the initial installation was quite a while ago and several upgrades have worked well in the meantime, it's now time to do a complete reinstall.
Unfortunately, the upgrade was a disaster. But the support in this forum was a pleasure again. - Thank you very much.
When reinstalling, I will definitely follow your tip regarding the default mirror.