[michaeldwan]

Failing in one AZ probably means they changed something with DNS servers for that region. We had a similar issue recently when we rolled out a new DNS server that returned longer SOA records which broke python's mysql driver in only some regions during the rollout. Debugging nightmare fuel.

[sk5t]

Yes, my theory was also some DNS variance in the AZ, but AWS stolidly refused to supply any information to that effect. Adding 'dig +trace' to the equation only deepened the mystery.