Having similar issues trying to play with a friend who i've played with since more than 5 years ago. I know for sure that my PC specs have worked well in MP before but my friend got a new monster computer with 32GB ram, latest AMD CPU and NVIDIA RTX 3090 with windows that might be causing issues. My system is Ryzen 5 3600 and GTX1660 with 16GB ram.
We did some pretty extensive testing having used the "remove all game data" from the launcher and with a fresh install of A20 experimental. Also made sure to have textures set to HALF on both systems to be sure. No matter what we try with EAC disabled and with the game as vanilla fresh as possible using vanilla profiles, the server side either:
1. Crashes as soon as the other player connects (Windows)
2. Has connection time out for the one connecting and then kicks the player (Arch Linux Steam with latest proton-ge-custom-bin, same Ryzen 5 3600 system (i dual boot))
Reading the logs from each connection attempt, there are no errors registered in conjunction with the connection. On my windows install the server immediately self-destructs after it says my friend successfully joined the server. Literally the last log line in the file, then dead
Log sample from this case:
https://justpaste.it/3ml26
Game also crashes in the same manner if my friend tries to host.
The crashing behavior is not 100% consistent. Most of the time the game crashes straight to the desktop, but i once got something that i think was the UnityCrashHandler showing for a while after the crash but not totally sure.
In single player everything works flawlessly for both of us.
Hope this helps.
Anything else we can try? In the past we have always been able to play after making sure EAC is off and using a public server with a password instead of creating a server which is only visible to friends on steam.