I usually don’t have to do this at all, but this is my system for the rare hiccups, in order.

• ProtonDB
• Are they using an incompatible anticheat?
• Different proton (usually one a few versions behind esp if it’s an old game, maybe GE, more than that isn’t useful)
• Verify installed files through steam
• Make sure my mod load order isn’t crap (if applicable)

.

• Check if the game launcher actually points to the correctly named file + rename / change the exe if not (usually this gives an error box saying the file is not present)
• Uninstall + reinstall (esp if previously worked)
• Broad internet search for game + linux distro
• Launch options I’ve used for other games (PROTON_USE_WINED3D11=1 often is my first tried)

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• Protontricks, tell it to use a different version of windows
• Uninstall DLC, launch vanilla once, reinstall DLC (yes this has worked more than once for various weird issues, no i don’t know why)
• Backup personal data files, + delete all installed files / workshop files + verify installed files again
• Wait for someone smarter than me to figure it out / a system update that addresses it / new proton and check in a few months