I renamed the prefs map to OLDprefs and got a new pref map, so maya looks like new again haha this made the trick for me, at first. Maya does now enable me to connect to the cutout opacity, but as soon as i start to render maya crashes. however creating the cutout opacity in the attribute editor as BldRnr told me to made the trick. my leafs are now also renderable and everything is working perfectly.

BIG THANK YOU TO EVERYONE FOR YOUR HELP