steamtechnical-issues
Every now and again Steam will decide to Show a unknown error popup when starting a game:
"Steam- Failed to start game (unknown error)"
The only way I can fix this issue is to restart Steam. This is troublesome because I am always in a voice chat when this occurs. I have no other programs running when this happens.
What could be the cause of this?
This appears to be a Steam problem, backup your data and reinstall Steam.
I've found out a way to fix this, go to the mu folder, for example:
C:\Program Files (x86)\WEBZEN\Mu
and make sure in main.exe, "Run this program in compatibility mode" is set as "Windows XP (Service Pack 3)", and "Run this program as an administrator" is set as-well
then click on Windows + R and write:
C:\ProgramData\WEBZEN\BrowserPlugIns
then, on CMStarterCore.exe do the same
Best Answer
If you are using a 32-bit operating system, it is strongly possible that the executable file is developed for the purpose on running on 64-bit, not 32-bit thus causing the error. Or it might also be possible that your computer can't recognise the .exe file as a 32-bit file, thus failing to start the game.
Another possiblity is that your DirectX may be outdated, thus preventing the .exe file from running. If this is the case, you can find out how to download the latest DirectX (DirectX 11) here: http://support2.microsoft.com/kb/179113
In addition, a lot of cases might be that your antivirus firewall is blocking the game from running. If you have any antivirus program on, you might want to turn them off.
Source: http://steamcommunity.com/app/299720/discussions/0/35221584612613774/?insideModal=1#p2