Well, one user fixed this, and it’s Comcast. Which led me to dig up the answer we got from Comcast last summer at a school for kids on the Autism Spectrum:
The problem was that “Smart Packet Protection” had been enabled on our Comcast System. This “protection” was an extra layer of blocking or security for Comcast customers. It was a feature that Comcast implemented on all customers, but had since disabled on all customers because there were so many problems with it. It had been “enabled” in our Comcast system when it was not supposed to be.