The game stopped working on Linux recently. I had to tinker for over 3 days to get it running again and want to share some tips.

I followed https://www.protondb.com/app/244210, clearing the compatdata prefix, selecting a recent Proton GE and letting it install dependencies. Note when people say these install in about 15-40 minutes, if you have an old hard drive the .NET installs may take a lot longer. They also seemed to just freeze a few times which didn't help me. I had to keep killing Setup.exe in the task manager and running the game multiple times.

I now have the game running well with Proton 5.0. I do not use Content Manager.

• One important thing I learned is be very careful if you want to do manual edits to any of the game's .ini files. Back them up first. Particularly a simple edit to launcher.ini seemed to stop the game launching at all with a confusing error about Steam being inactive. It was not. (Note also these error message boxes wouldn't even show up on Proton GE. Switch to Proton 5.0 to see them). Putting the old launcher.ini back seemed to fix this (If that doesn't work for you I would just make sure Steam is up to date, rename the "Assetto Corsa" folder in Documents and reinstall the whole game).
  
  
• The other tip is if you run the game and the launcher crashes out after loading the cars and tracks and just before showing the main menu, this may be because it cannot get video resolution information from dxgi.dll. Run with Proton 5.0 and if you get an error pop up saying "Module not found" for acVideoModes.dll, then run protontricks for 244210, winecfg and add/edit an override for dxgi.dll so it says "native, builtin" (I had installed DXVK which may be what caused this. Not sure). If you still get the same problem, try installing vcrun2013 with protontricks as well.

Hope this helps someone. I wanted to stick this on ProtonDB but they want too much of my data!