Do you have python3 installed?

Yes, python 3.6.5-3ubuntu1

Can you run any proton-enabled game and then check /tmp/dumps/$USER_stdout.txt? The last few lines should potentially contain some information if proton crashed

In /tmp/dumps I haven't find $USER_stdout.txt file. But in latest crash***.dmp file I see string Assertion Failed: Standalone evaluator returned error code for app 6900 ) installscript_posix.cpp:416

Sounds like a problem running the proton script. Do you have a "python3" in your $PATH? E.g. does "python3" do something in a Terminal? What happens when you run "python3 proton" in the Beta directory?

When I type this command:

~/.steam/steam/steamapps/common/Proton 3.7 Beta$ python3 proton
Proton: No compat data path?

Very strange. I opened file proton in Proton 3.7 Beta directory and see the comment:
#To enable debug logging, copy "user_settings.sample.py" to "user_settings.py"
After I did it I run the game and script made dist directory with all needed files in Proton 3.7 Beta cat

If I run python3 proton in terminal I still get

Proton: No compat data path?

Any ideas?

Yes because running directly it's expecting a path to store the prefix (or path to existing prefix)

I have python3 in my path set and executing it gives me a shell stating Python 3.6.6 which I have to exit()

I change the name of the debug file to user_settings.py
cat user_settings.py

#to enable these settings, name this file "user_settings.py"

user_settings = {
    #logs are saved to $HOME/steam-$STEAM_APP_ID.log, overwriting any previous log with that name
    "WINEDEBUG": "+timestamp,+pid,+tid,+seh,+debugstr,+module",

    "DXVK_LOG_LEVEL": "info",

    #Enable DXVK's HUD
#    "DXVK_HUD": "devinfo,fps",

    #Use wined3d for d3d11 instead of dxvk
#    "PROTON_USE_WINED3D11": "1",

    #Disable d3d11 entirely
#    "PROTON_NO_D3D11": "1",

    #Disable in-process synchronization primitives
#    "PROTON_NO_ESYNC": "1",
}

I commented everything in. Stopped steam. started it from command line moved through the UI and pressed the play button again. Thats the commandline output I received.

installscript_posix.cpp (416) : Assertion Failed: Standalone evaluator returned error code for app 292030
installscript_posix.cpp (416) : Assertion Failed: Standalone evaluator returned error code for app 292030
Installing breakpad exception handler for appid(steam)/version(1536436120)
assert_20180923222024_24.dmp[10212]: Uploading dump (out-of-process)
/tmp/dumps/assert_20180923222024_24.dmp
GameAction [AppID 292030, ActionID 1] : LaunchApp changed task to SynchronizingCloud with ""
GameAction [AppID 292030, ActionID 1] : LaunchApp changed task to ProcessingShaderCache with ""
assert_20180923222024_24.dmp[10212]: Finished uploading minidump (out-of-process): success = yes
assert_20180923222024_24.dmp[10212]: response: CrashID=bp-cfbbc2e7-a0e8-4bbb-a5eb-775d22180923
assert_20180923222024_24.dmp[10212]: file ''/tmp/dumps/assert_20180923222024_24.dmp'', upload yes: ''CrashID=bp-cfbbc2e7-a0e8-4bbb-a5eb-775d22180923''
GameAction [AppID 292030, ActionID 1] : LaunchApp changed task to SiteLicenseSeatCheckout with ""
GameAction [AppID 292030, ActionID 1] : LaunchApp changed task to CreatingProcess with ""
GameAction [AppID 292030, ActionID 1] : LaunchApp waiting for user response to CreatingProcess ""
GameAction [AppID 292030, ActionID 1] : LaunchApp continues with user response "CreatingProcess"
Opted-in Controller Mask: 70
Game update: AppID 292030 "", ProcID 10214, IP 0.0.0.0:0

I was reading answer 10 of this steam forum thread which had a similar error, convincing me, that it is indeed a rights issue. To solve it I tried the following.

1. I installed a second smaller game, in my case, starbound from my Steam UI -> Library as a reference
2. tried running it and checked for the same error message, reproducing the error
3. copied proton 3.7 and starbound to a ext4 partitioned folder
4. created two symbolic links in my ntfs partitioned Steam folder one linking proton 3.7 and the other starbound
5. made in in .../steamapps/common/Starbound/linux / run-client.sh and starbound executable chmod a+x ... (for both filenames)
6. did hit the Play Button in Steam UI -> Library -> Starbound
7. game launched

Trying the same with The Witcher 3 was fairly harder

1. copied The Witcher 3to a ext4 partitioned drive
2. created a symbolic links in my ntfs partitioned Steam folder one linking it
3. after copying I realized that for some reason all files in The Witcher 3 folder had file and group root
4. changed permissions for all files sudo chown -vR username:usergroup .../The\ Witcher\ 3/ note: (usergroup was in my case username again)
5. made all files in .../The\ Witcher\ 3/bin/ executable chmod a+x *
6. did hit the Play Button in Steam UI -> Library -> Witcher 3
   unfortunately no success, The Permission denied error message was gone (I guess this should be enough for this bug), but game didin't start. Instead the was a small Steam Update Window

subprocess.CalledProcessError: Command '['/home/user/Proton 3.7/dist/bin//wine64', 'winepath', '-w', 'Witcher 3']' returned non-zero exit status 1.

1. Steam UI -> Steam -> Settings -> Steam Play -> Compatibility Tool -> Proton 3.7.6 Beta
2. Steam restart
3. game launched

Summary what I have gathered together so far:

• the mounting options for the ntfs partion desscribed in #1568 doesn't offer sufficient rights
• contrary to the answer 10 of this steam forum thread my Proton 3.7.6 Beta is installed on a nfts partion (cause I only moved and created a link for the Proton 3.7 folder, I realized this only after Witcher 3 was already running)
• after copying files to a ext4 partion, one has to take care that all permissions and executables are set properly by hand
• would be nice if the steam downloader, would at least for now discourage downloading games on ntfs partions