mDutch:
My fade works perfectly if I use a fadeInterval of 1000(milliseconds) combined with a delay of 1000 in my loop.
What happens if you take the delay() out of your program?
My guess is that the delay(1000) is conflicting with a millis() interval of 1000. After the delay() the entire period of the 1000 millis() interval will already be used up,
If you need the wait caused by delay(1000) then you should also implement that using millis().
...R