Okay, so I think now I've got a real strange problem.
I have set up a project, that consists of an editor timeline, in which there is a comp shot. Inside the comp shot, there is background footage, and two images, (ball 1 and ball 2) flying around on it. Now, inside the comp shot they view perfetly, everything is fine. But, on the editor timeline, ball 1 is only visible in every second frame - in the right position, but every second frame it's invisible. then, when I render the project out into a .mp4 file, ball 1 is not visible at all!
Funny thing is, ball 2 (it is really much set up exactly as ball 1, only with different transform keyframes), stays perfectly visible all the time.
Also, If I turn the motion blur off on ball 1 the problem is solved - but I really want motion blur, it looks so cool!
One way that worked is to first export the comp shot into an image stream, and place THAT on the editor timeline, but that's really time-consuming and unnecessary...
Any ideas on how to fix it, or is this a bug that has to be solved by the developers?