SERVFAIL with deutsche-glasfaser.de

[Bucking_Horn]

Maybe. Not for all requests, though - it seems only DNS requests to their own authoritative DNS servers are not working.

Could you also try the dig to their IPv6?

[firewire]

I activeted IPv6 support in my router and now it works:

Thank you all for your support!

[deHakkelaar]

Nice.
Unbound, like most all software, prefers IPv6 over IPv4.
It still means something is wrong upstream for the IPv4 part though