So I tested the Linux build on the Steam Deck this morning, and it seems to work fine there.
I realized an easy way to find the logs is to go to the festival, click C for camera, N for screenshots, then click Open Folder. Then go up one folder to find the save data folder which should have the logs. Not sure if they'll be handy but it's worth checking.
Otherwise I don't really have a good hunch for what the problem is yet. I wonder if Opensuse comes with a restrictive firewall by any chance? If your other multiplayer games work fine than maybe not, but perhaps Opensuse has some more strict settings about unfamiliar executables downloaded from the Internet? If you feel comfortable with it, might be worth disabling the firewall temporarily just to make sure that's not the problem?
Sorry I'm not sure why this is happening yet! 😔