Mars.exe Error / invalid game root directory

[lordxms]

Every time i try to launch the game, when i click on play in the launcher i got an error message related to mars.exe.

Here is the log :

T (2014/07/26 14:55:10) [app] MARS2 32-bit Retail Environment 2.0.25092.5
T (2014/07/26 14:55:10) [app] Last built on Dec 23 2013 22:32:28
T (2014/07/26 14:55:10) [app] CWD:
T (2014/07/26 14:55:10) [app] F:\STEAM II \SteamApps\common\Sword of the Stars II\bin\x86
T (2014/07/26 14:55:10) [app] Args:
T (2014/07/26 14:55:10) [app] F:\STEAM II \SteamApps\common\Sword of the Stars II\bin\x86\mars.exe
W (2014/07/26 14:55:10) [app] Invalid game root directory:

Spec :
Win 8.1 pro / 64 bits
GPU : AMD HD 7870
CPU : Intel i5 4570
All drivers updated.

Can someone help me ? I just bought the game and cannot even launch it !

Thanks !