We detected bad glue being issued by your name servers. Either required glue was missing or the glue returned contained incorrect IP Addresses.
Glue refers to additional DNS A records that are returned to assist in locating name servers.
Additional Information
The DNS check requests NS records for the requested domain starting from the root servers and continues following referrals as long as the answer returned is not marked as Authoritative. Whenever a name server issues referrals, it can additionally include A records which contain IP Addresses for the hostnames in the referrals. In some cases these records are required, such as when the requested information is contained in the zone needed to resolve the hostnames, and also sometimes optionally to increase DNS performance.
We perform a new lookup from the root for each of these A records and make sure that they contain valid, correct information. If the IP Address given to us as glue is incorrect, we will show you this warning.
Configuration of glue records is covered in RFC 1033