I'm getting the same error.
OS: Solus
lsb_release -a
LSB Version: 1.4
Distributor ID: Solus
Description: Solus
Release: 3.9999
Codename: shannon
uname -a:
Linux ezyrhinox 4.17.14-86.current #1 SMP PREEMPT Sat Aug 11 13:47:11 UTC 2018 x86_64 GNU/Linux
gcc: 7.3.0
nvidia drivers: 390.77
1080Ti
Same issue here on Arch Linux: ErrorLog_08-22-2018__01-36-11pm.txt
This didn't happen when I was running Steam through Wine.
Same issue on arch with nvidia
If it helps, my log http://ix.io/1kUr
Arch AMD Mesa
Problem is introduced with Steam native. use Stream runtime and it works.

Got the same problem on Antergos but then I got it to work by using Steam runtime instead of native and adding +r_renderAPI 1 to the launch options.
Mine started working after I turned 'Linux Steam Integration" (LSI) off. I then had to set the +r_renderAPI 1 as a launch option.
steam-runtime fixes it weirdly enough
issue is closed ?
I still have it, I tried a lot of config and still "Failed to read default.cfg".
I tied: native / runtime and the 2 availables PROTON version.
Do you think it can be related to my MESA 19-dev build ? It work with other game and this error seams not to be related with mesa...
Hello @cclecle, this issue is currently open as indicated directly below the title at the top of the issue report.
Hello kisak, sorry I misread the tag..
So I did some more investigations. Apparently it is related to mesa 18.3 OR the way I link mesa 18.3.
MESA 18.3 + GLX 18.3(radeon_si) => ERROR default.cfg
MESA 18.3 + VULKAN (RADV 18.3) => ERROR default.cfg
MESA 18.1.9 (the native one in my debian) + VULKAN (RADV 18.3) => Launch and Run OK
MESA 18.1.9 + VULKAN (RADV 18.1.9) => KO, but it is a known issue
MESA 18.1.9 + VULKAN (AMDVLK 18.40) => Launch and Run but freeze a few seconds after
So th e default.cfg is linked to MESA 18.3 usage, even if it is not really used (VULKAN rendering case).
Does this issue still occur on latest drivers/proton?
It seems to work fine now with "Proton 3.16-9 selected by Valve testing".
By the way, even though I started Steam with STEAM_RUNTIME=0, it's giving a bunch of messages about STEAM_RUNTIME_HEAVY. Not sure if that's related.
proton 3.16-9x1 2019-07
Crashes on launch with an error box that says failed to read default.cfg.