@rg305, I'm not sure if this could be a reason, but do you remember one user who is using AWS and several of his IPs stopped working? AWS was dropping packets which made it seem like it was a firewall blocking. In the last post (#6) the user included a reply from AWS.
To quote from end of his post:
Quoting from aws support:
"With the size and scale of AWS, traffic from different IPs can take different routes within the AWS network and a routing issue could cause a packet drop which looks very similar to a firewall that simply drops denied packets"