Delegation check failed

Hello dear community,

I own an .at domain and changed the namesservers from my registrar to the nameservers of dynv6. In dynv6 I’m on single zone.

Delegation check shows following error:
failed to get NS records for at.: read udp [2a01:4f8:1c1c:4c96::]:37983->[2001:62a:4:321::84]:53: read: connection refused

Does anybody know what’s this about?