Hello! This issue has been pestering me since one year. The issue appears on different computers that I use (one desktop with Win 7 and one laptop with Win 8).
Every time that I exit C2 and leave the Steam open, and than want to reenter it, ie. launch again C2, Steam complains with Failed to start game (app is already running).
This issue is all the more annoying, since I cannot close Steam, since it waits for ever for C2 to shutdown.
And Since I develop a plugin, it becomes a total nightmare to shutdown Steam via Windows taskbar, 'cause I need to relaunch C2 in order to see the effects of the plugin modification.