This guy had the same problem back in 2023: https://community.cloudflare.com/t/protected-web-socket-connection-dies-after-30-secs-cloudflare-ipv6-issue/547127.
He’s right, it’s an IP6 thing. The three IP6 addresses for ai.online-go.com all fail 20s after connecting. The three IP4 addresses are all fine - I could keep sending until I got bored.
Connecting via a VPN (to London, Amsterdam or New York) also solved the problem, presumably because it no longer relied on BT connecting to Cloudflare, or maybe because it was IP4 at the crucial point.
I’ll see if EE (my ISP = BT) can help, though it’s going to be fun trying to get something so technical through the front-line “Try turning it off and on again” guys. 