Cowboy is quite a well respected we server of the Erlang flavor. I'd guess heroku rejiggered something in their stack, perhaps adding cowboy as a reverse proxy

Cowboy is quite a well respected we server of the Erlang flavor. I'd guess heroku rejiggered something in their stack, perhaps adding cowboy as a reverse proxy or load balancer in front of their junk. Cowboy apparently shot yor no-good dirty sidewinding web requests in the face.