Emptyict, what type of graphics card do you have? My first guess is that desktop effects are enabled, but your card/driver combination doesn't support them.
By default, KDE enables desktop effects. The failsafe session should log you in with desktop effects disabled - this is your chance to open System Settings and try disabling them. Log out cleanly, then try logging in normally. (If everything works after that, you can experiment with enabling XRender effects or basic OpenGL effects.)