I have EvoX and by pushing both trigers + start + back it should restart the xbox or go back to the dashboard ...
but sometimes it doesnt work... the screen keeps just beeing black and I have to go to my xbox and turn it off/on ...

Is there any way to fix that?

Depends, do you have an x2 bios? do you have that IGR turned off? If using from a game I hear its better to pause the game first too.