Sanctum only starts if you use the PROTON_USE_WINED3D11 environment variable.
Steam - System Information
Hello @llikeitt, you may have better results with mesa-git and llvm-svn from the AUR.
Now it works without the PROTON_USE_WINED3D11 environment variable.
New Steam - System Information
For me it has been running out-of-the box since day one.
System info
PROTON_USE_WINED3D11
Sanctum only starts if you use the PROTON_USE_WINED3D11 environment variable.
Steam - System Information