Okay, I figured it out, it's the screen resolution. I tried 1280x720 (close to that of the Steam Deck) and 1600x900, and in either case the error does not occur. It does occur at 1920x1080 and above though. That explains...
Apparently the game works fine on Steam Deck, so uh... what? Is something wrong with my system? https://twitter.com/bibresin/status/1595963299191222273
I still have no idea what the root cause is, but it's probably related to this at line 4431 of the Proton log: [code] "メモリを確保できません" means "Cannot allocate memory" in Japanese. (the system language/locale is set to Japane...