I forgot to mention i also have a issue ticket at https://github.com/UnderminersTeam/UndertaleModTool/issues/2331
Hello @ArchoDarko, this game has a Linux native variant, and if there's no Proton log in your home folder when you set the game's launch options to PROTON_LOG=1 %command%, that implies that the native variant is in use and Proton isn't involved with the game.
For clarity, please go to the Library view in Steam, select the game, then click the ⓘ button on the right and verify which variant of the game you're using. Also, please describe the startup sequence that occurs with this mod. It's not immediately clear if everything is running with Proton, or in the same compatibility tool environment. The issue report you linked hints it isn't and that might cause additional complications.
I would use the linux native version, but for some strange reason, it uses gamemaker's YoYo compiler, which embeds the code into the executable instead of the data.win file (as is standard for gamemaker games), which undertalemodtool doesn't support viewing and editing code for. Also, i can confirm i am using Proton Experimental for this game via the ⓘ button, but i'm unsure what you mean by the startup sequence for the mod.
proton experimentalx2 2026-03PROTON_LOG=1x1 2026-030x80004005x1 2026-03
Compatibility Report
System Information
I confirm:
I unfortunately can't locate any file named 354240.log, so i can't attach it here
Symptoms
I was using UndertaleModTool (which is a tool to mod undertale, however any gamemaker game will work), to mod please don't touch anything, and whenever i try to use the "Temp run game" tool from the "File" menu, it gives me the error:
Looking online, I've found that this error is caused specifically by using wine/proton for gamemaker games. I'm well aware that valve has nothing to do with undertalemodtool, i've just seen a few people report other Graphics_DisplayM.cpp issues here.
Reproduction