Hi Oli
The most likely reason: Bind 9.11 enables EDNS cookies by default, but the authoritative servers for this domain do not handle EDNS correctly:
https://ednscomp.isc.org/ednscomp/b01039e111
quick fix: server NSNAME { send-cookie no; };
That was the cause and the fix. Thanks a lot!
Mit freundlichen Grüssen
-Benoît Panizzon-