Losing internet connection on high (but not excessive) load

Hi,

I hope I'm posting this in the right place, I couldn't really find a better place for it on these forums but feel free to move it or suggest better places to ask my question.

I'm writing a plugin for a minigame together with 2 other people who live far away, we only communicate online. We don't have a "real" server yet but, for testing, we are occasionally hosting on our own computers via ngrok or playit.gg.

For me hosting on my Mac, no problems, great performance, thousands of mobs on the screen with 5 players connected over ngrok and everything still runs smoothly. A friend with Linux has no problems either. He also has a Windows machine which works fine too.

My other friend is not so lucky, though. Everything used to work just fine on his 6 year old PC, Windows 10. But now he bought a brand new, high spec gaming machine. Watercooled Intel i9 12900k, Watercooled RTX 3090 Ti, 32 GB ram, 2 ssds, etc... Windows 10 like the old one. But whenever he's hosting and playing with friends, when he gets to a phase of the game where a lot of mobs are on the screen (but much less than what his old PC could handle), his internet connection just dies. The server itself continues to run, he can still win the game he's playing, not even any hiccups, but everyone else is disconnected and he can't connect to the internet anymore at all. No webbrowser, no e-mail, nothing. On restart, he gets a blue screen with a sad smiley and he has to physically power off and on. Then everything works again. Other people in his household don't notice any connection problems.

The minecraft server log only has messages for all players saying "playerName lost connection: Internal exception: java.io.IOException: An existing connection was closed by the remote host" but nothing else.

The computer is brand new, he hasn't installed any crapware, just plain Windows 10. It was built to order by a company called "alternate". He normally has no problems with his internet connection, only when hosting our game.

Does anyone have any idea how we can troubleshoot this and figure out what's going on? I have a lot of programming and systems experience on Mac and Unix/Linux, but limited knowledge about networking and none whatsoever on Windows so I don't know where to find log files, what utilities to use, etc.

Our plugin uses viaversion and protocollib to send packets, drastically reducing server load because the server doesn't even know about the mobs, we're managing them all ourselves. It is a lot of packets, but if his 6 year old PC could handle it, I don't see why the new one can't.

Thank you for any ideas or tips you might have!

Michel