Nameserver Predelegation Check fails: Unexpected RCODE 901

The Nameserver Predelegation Check for the dynv6 nameservers fails for my domain

output looks like the following:
|Error|901|Unexpected RCODE (target, entity, RCODE)

Doing the check with the punycode representation (henninghä yields exactly the same output.
However for other nameservers this works fine.
Since this a requirement for some domain providers to update the name servers it would be cool to get this fixed.
Is this really a nameserver issue, if yes: can it be fixed in the foreseeable future?

Thank you very much in advance!