The game is now playable out of the box with Proton.
There is one remaining issue: Game always launches minimized (this appears to be a pretty common problem with UE3 games). You have to click up the games screen (it will be completely black) and then hit alt+enter twice to get video.
Games option screen forgets your resolution setting (but game is actually displayed in the resolution you have previously set), this is however not a Proton issue the game has that bug on Windows too.
Included a log of launching the game, bringing up the screen and alt+tabbing two times, and then playing for a bit.
Compatibility Report
System Information
Distro: Linux Mint 19.1 Tessa (64 bit)
GPU: Vega 56
Driver/LLVM version: Mesa 19.1.0-devel/9.0.0
Kernel version: 5.1.0-050100-lowlatency
Proton version: 4.2-3
I confirm:
Symptoms
When launching the game, a splash screen/loading window displays for a few seconds and it closes and nothing happens.
steam-263520.log
system_info.txt
Found this in the log:
"3044.274:0028:0029:warn:module:load_builtin_dll cannot open .so lib for builtin L"Steam2.dll": /mnt/Games/SteamLibrary/steamapps/common/Proton 4.2/dist/lib/wine/steam2.dll.so: cannot open shared object file: No such file or directory"
I think this is another game with the Steam2.dll issue: #2613
Reproduction
Just launch the game