It seems the game doesn't like that path. However, generally, you given it's the portable version it should work in any path. Do you have any special characters in the install path of the game? Can you move the path in itch.io?
There are no special characters in its path. Just to be clear, I installed the game successfully via the Itch.io app, but it fails to start from within the Itch.io app. However, it works if I manually navigate to where the game is installed by Itch.io, and the game starts normally when I double click on the executable. So it sounds like a problem with the Itch.io app?
Yes, it might have something to do with the itch.io app if you are able to start the game directly. However, I assume you are successfully able to start other games!?
What you could try (not sure if that's the case?) to move the itch.io app/game to a directory with only ASCII characters. E.g. from "C:\Über\..." --> "C\Uber\...". Not sure if that's where your games reside? If you have only ASCII characters for the game's path, it's odd that that error message would happen however. Maybe try to talk with the itch.io team as well?
Yes, for some reason I'm able to start other games without this error from within the Itch.io app. And I'm pretty sure my entire path is ASCII only, so I'll probably feedback to the Itch.io team, then. Thanks for your help!