All domain profiles suddenly stopped working


last week we installed Univention on a VM to test it out (we want to replace Windows Server 2016 with it) and everything worked for the most parts.
This week we came back to work and wanted to continue testing it, but suddenly I get the error message “We can’t sign into your account”.
We made a backup last week for a case like this, restored it and… the problem still consists.
We then ran the system diagnosis tool and it gives a warning about how the DNS server doesn’t have A/AAAA records (which it has and also works, otherwise we wouldn’t be able to connect a PC to the domain)
We can’t find any errors in the logs.

Any ideas what we could do?