Is this a new Steam installation? Steam Beta? Upload a gist with your system information, please.
I've never heard of the SteamApps folder being created by Steam for Linux with that casing. Did you do something special to create the SteamApps folder, like copy it from a Windows installation?
Old steam installs can have a camel case SteamApps folder that's been grandfathered in and remained functional.
I see, looks like a Steam client bug then. We don't create that path in Proton, we just pass through what the Steam client hands us in $STEAM_COMPAT_DATA_PATH. Thanks for reporting.
The code is supposed to check for that, so is it possible that /home/user/.local/share/Steam/steamapps/ also exists on your machine?
I have same issue with all games. I tried changing Proton Version and reinstalling Steam.
Quick&dirty hack got me to being able to play my games (Dishonored 1+2, Mass Effect 1+2) again:
cd ~/.local/share/Steam/ && mv steamapps/* SteamApps/ && rmdir steamapps && ln -s SteamApps steamapps
Would probably work the other way around, too.
It seems they changed cases ... old (for years) installation says "SteamApps", but proton script is searching for "steamapps" now.
On case-sensitive file system that is a problem.
Compatibility Report
I confirm:
Symptoms
Steam client can't start KOFXIII.
Here is the reason:
Traceback (most recent call last):
File "/home/user/.local/share/Steam/SteamApps/common/Proton 4.2/proton", line 325, in
with prefix_lock:
File "/home/user/.local/share/Steam/SteamApps/common/Proton 4.2/filelock.py", line 322, in enter
self.acquire()
File "/home/user/.local/share/Steam/SteamApps/common/Proton 4.2/filelock.py", line 251, in acquire
self._acquire()
File "/home/user/.local/share/Steam/SteamApps/common/Proton 4.2/filelock.py", line 383, in _acquire
fd = os.open(self._lock_file, open_mode)
FileNotFoundError: [Errno 2] No such file or directory: '/home/user/.local/share/Steam/steamapps/compatdata/222940/pfx.lock'
Steam client did not make "steamapps" directory, it made "SteamApps" directory. The files are in '/home/user/.local/share/Steam/SteamApps/compatdata/222940'.