Hello
The .is domain provider is emailing every night and saying "dns-server ns13.zoneedit.com is not properly registerd in dns (ns13.zoneedit.com => 162.220.33.236 => t1.zoneedit.com)"
We had to change the glue record for ns13 because of a DDoS attack, the .is registry is probably expecting it on it's old IP. We'll revert it later today.