Played games using Proton Experimental earlier today. Got an update around 20 minutes ago and can no longer launch Path of Exile 2. Fails with the python error below.

Distribution: Gentoo Linux

steam[1252]: Adding process 57384 for gameID 2694490
steam[1252]: Traceback (most recent call last):
steam[1252]:   File "/home/user/.local/share/Steam/steamapps/common/Proton - Experimental/proton", line 1869, in <module>
steam[1252]:     g_session.init_session(sys.argv[1] != "runinprefix")
steam[1252]:   File "/home/user/.local/share/Steam/steamapps/common/Proton - Experimental/proton", line 1776, in init_session
steam[1252]:     g_compatdata.setup_prefix()
steam[1252]:   File "/home/user/.local/share/Steam/steamapps/common/Proton - Experimental/proton", line 1175, in setup_prefix
steam[1252]:     setup_openvr_paths()
steam[1252]:   File "/home/user/.local/share/Steam/steamapps/common/Proton - Experimental/proton", line 333, in setup_openvr_paths
steam[1252]:     contents = json.load(file)
steam[1252]:   File "/usr/lib/python3.9/json/__init__.py", line 293, in load
steam[1252]:     return loads(fp.read(),
steam[1252]:   File "/usr/lib/python3.9/json/__init__.py", line 346, in loads
steam[1252]:     return _default_decoder.decode(s)
steam[1252]:   File "/usr/lib/python3.9/json/decoder.py", line 337, in decode
steam[1252]:     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
steam[1252]:   File "/usr/lib/python3.9/json/decoder.py", line 355, in raw_decode
steam[1252]:     raise JSONDecodeError("Expecting value", s, err.value) from None
steam[1252]: json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
steam[1252]: Game Recording - game stopped [gameid=2694490]

Switching Proton-9.0-4 and the game starts successfully.