Just wanted to add that I'm testing this game with each new beta of Proton. Still no solution has been found as of the current 3.16-7. I still get the same error as mentioned above. I won't post again unless it gets fixed or a new error message pops up.
Same problem here (Arch Linux).

0026:err:module:import_dll Library MFC42.DLL (which is needed by L"C:\users\steamuser\Temp\capb530.tmp\9.csx") not found
wine: Unhandled page fault on read access to 0x49000004 at address 0x7bc49391 (thread 0026), starting debugger...
mfc42.dllx1 2019-04
Compatibility Report
System Information
I confirm:
that I haven't found an existing compatibility report for this game.
that I have checked whether there are updates for my system available.
Steam log: https://pastebin.com/2CZa7tU3
It was so long that github wouldn't allow me to upload it.
Symptoms
I get a runtime error with the message: Failed to load plugin C:\users\steamuser\Temp\capbde5.tmp\9.csx (126)
There is no game window that launches. It runs in the Construct game engine (not sure if it's 1 or 2. I believe the dev said 2). I'm not really sure how to fix it.