Actually this is a known problem, "Malformed Response". But that's all I know. I think it is an issue somewhere with the connection between your server and ours, it isn't our server itself that is the problem. Only a few people seem to get it.

If you know how to use wireshark, maybe you could capture the request (what TPT sends and how the server responds). If you do, send that privately, it would contain stuff that would let me login as you.

Although I did just check the website code. I do see one situation where it would return HTTP 200 with no other response, but that is only if you were already authenticated and the request was done incorrectly. Maybe restarting the game or deleting powder.pref would fix it.