Looks like the actual response says:

> Your browser sent a request that this server could not understand.

> Reason: You're speaking plain HTTP to an SSL-enabled server port.

> Instead use the HTTPS scheme to access this URL, please.

Since this only happens on HTTPS, it's a bit hard to MITM to figre out what's actually going down. However, Wireshark shows some interesting stuff: Emacs seems to complete a TLS handshake, then after some packets, sends a TCP RST. Then, astonishingly, Emacs sends a plaintext HTTP request _on port 443_ to the server.

I can provide the Wireshark pcap if needed, though I doubt it's too helpful. Let me know if there's any other way I can help debug.

Thanks,

Christopher Cooper