Have you done any DNS migrations the past couple of weeks?
I have tried resolving webs.com from various nameservers and only 1 set gives me SERVFAIL. The resolvers my ISP owns.
My own resolvers, Google DNS, Level3 DNS and direct lookups works out beautifully, just like they should.
I have seen this behavior before once. The problem seemed to be that the zones were migrated from one DNS setup to another, and that the new DNS setup used lower serials than the old one. It caused a lot of resolving nameservers to get really mad and refuse looking them up, even after changing the serials to being higher than the old ones.
I never actually got to know from the ISP that I contacted exactly what they did to their resolving DNS setup to get it working again.
Your current serial in SOA is "11434991" if you have migrated and your old provider used the typical YYYMMDDSS (SS being serial) then the new serial is in fact lower than the old one would have been.