So i've tried reinstalling this build several times, enabling and disabling 3D Accelerator etc and nothing worked. Now i found a solution. It turns out i couldn't open the start menu because i've disabled most of the start menu options in the settings menu. I'd assume it's a bug within build 10125. Any how, "Store and display recently opened programs in start" must be enabled. At least for me.
Well even, a well-finished RTM product can be buggy, that's why we receive hotfixes through windows update. And this is only a preview, so bugs are sure to be there.