[Error] Failed to start game (unknown error)

I've been unable to play RO2 for many months.. got an update today and hoped it might help so i tried again. No joy.







I'm not sure what to do. All other Steam games work fine. The troubleshooting suggested in the link provided in the error fail to help.



I haven't deleted the game & reinstalled... considering the game is 23GB that is not ideal. (Obviously I have verified the files numerous times)



What can I do?



Thanks.



EDIT: It is the Rising Storm/Red Orchestra 2 Multiplayer version on Windows 8.1. Everything on my OS & game are up to date.



I have deleted the two different config folders. No joy.



I've deleted the game exe & verified. No joy.