I did try the game on a another machine using Proton 6.3-6b. This system has a Radeon RX 480 and runs Mesa 21.0.3. With this installation I was able to complete the game. Up to now I did not notice any problems.
System information
Log
Edit: On Haswell it works a bit better when I use PROTON_USE_WINED3D=1.
Hello @friedc, there's a good chance you're seeing a video driver issue. While mesa does provide some of the Vulkan spec with the Intel Haswell chipset generation, that Vulkan support is incomplete, making the hardware unsupported by Proton in general.
Compatibility Report
System Information
5.4.0-81-generic [#91](/issue/ValveSoftware/Proton/91)-UbuntuI confirm:
steam-1097200.log
Symptoms
Crashes on the loading screen at 42%, an unity error message pops up.

Reproduction
Start the game.