Hey Tep,
just tried your circuit (again). Like before, the opto seems to function(via LED) but my arduino read nothing. I have already tried different resistor combos with the (working)cny17-4. As far as I can tell, it seems pretty forgiving, midi data gets through every time.
There has to be some hardware problem going on here. It doesnt read midi even when the opto is bypassed and the midi connected directly to the arduino. THis should work. There is no need for any kind of filter or anything, is there?