Fatal Error: Another Instance is already running..

only what i can think off this that some other app have same GUID

Recently a few players had a similar issue where their Steam client was either running the beta or outdated. Since you were able to play the game a few days ago this might not be the case. Check your Steam client is updated (Help -> About Steam should say Built Jun 8 2021 or similar) and that you are not running the beta.

Start by verifying integrity of the game files. If that doesn't work try reinstalling the game.

can be yes that this issue is with Steam. i todo little test and app self use simple inbuilt unity Mutex protection for avoid run 2 or more same games at same time. soo i guess what happens in here with Steam is that it opens 2 games either at same time or some other strangely mode

also that GUID what i talked earlier, as this is very specific for this game there no way that there some other app run at same time with same GUID.

Edited June 29, 2021 by gabberworld

This happens when you pin the icon to the taskbar in my experience with this game. Launch from Steam games list or desktop icon and seems to be fine.

I too am experiencing the same issue, I also have an AMD Ryzen 7, but no other specs in common. The game launches reliably (ableit slowly) once steam is fully closed, and is booted in the game launch process. This only applies to ONI, and no other games in my library.