Whenever I choose Skirmish, set all settings and start it, the game crashes with some Windows error that some read progress couldnt be performed.
Im using WIn7 64 Bit and the Campaign Missions work perfectly.

You're most likely missing activities definitions. Try reinstalling.

I installed it in some other location, now it seems working :>

Maybe it was in Program files earlier.

His symptoms match activities being missing, crashing not on startup but as soon as you choose a difficulty.