don't use the new proton_experimental. while it works for FA from steam FA run from FAF fails; this is what you'll get if you try to run it with proton experimental
On another note I'm getting another issue. On my new install of ubuntu 20.10 and with proton 5.13 (in which case the nice error goes away), but my FA stops immediately with no log output as you can see :
2021-01-20 03:53:57.860 INFO 396709 --- [ Thread-24] c.f.client.fa.ForgedAllianceService : Starting Forged Alliance with command: [/home/t/faf/run, /home/t/.faforever/bin/ForgedAlliance.exe, /init, init.lua, /nobugreport, /log, /home/t/.faforever/logs/game_13674228.log, /gpgnet, 127.0.0.1:13904, /mean, 1261.25, /deviation, 93.3293, /savereplay, gpgnet://127.0.0.1:45827/13674228/tatsu.SCFAreplay, /country, FR, /clan, BPT, /numgames, 2373] in directory: /home/t/.faforever/bin
wineserver: using server-side synchronization.
2021-01-20 03:53:59.067 INFO 396709 --- [pool-3-thread-5] com.faforever.client.game.GameService : Forged Alliance terminated with exit code 53
2021-01-20 03:53:59.391 INFO 396709 --- [ Thread-24] c.f.client.fa.relay.ice.IceAdapterImpl : ICE adapter terminated normally