Cannot start the game via Steam

[RavenDark]

Hey everyone,

now I finally bought GC at Steam, however I cannot start it. Every time I press the play button a popup opens, telling me "Failed to start game (app already running).".

Okay, so I checked the task manager but there is no gearcity.exe to be found. I then tried several other steps, but none of these worked either.

- verified game cache - no errors found
- deactivated anti virus (Avast)
- disconnected flight stick
- reset video settings
- manually installed OpenAL
- activated the stable branch
- deleted the appcache folder in the Steam main directory
- uninstalled Gear City Beta 1.13 which I still had installed
- restarted Steam and the PC several times

I still get that popup appearing as soon as I try to start the game. Obviously something makes Steam believe that GC is already running, but I have no idea what that could be.
Maybe some leftovers from the old beta in the registry? The log file only says "Looooooog".

Any ideas of what else I could try? Thanks.


Edit:
I'm using Win 7 64 Ultimate. Steam is installed on drive E, while GC is installed in a Steam library on drive G.

Edit 2: Downgraded to legacy branch, but still not working.