you have used white rectangles to hide some things and then you have placed other images on top of them. The white rectangles are being rendered in your png sequence.
You can open your scribe and change your background color to green (or any other color besides white) and you can clearly see where the white rectangles are going to cause a problem.
The solution is to remove the white rectangles and instead jump the camera to a blank area on the canvas.
Keep in mind that animation times (or possible pause or transition times of 0.0 seconds may currently cause other problems (bugs) so if you run into any problems with timing or images not showing up, 0.1 sec times may need to be used in some cases instead of 0.0. (I don't know if it is just pauses or just animation times or just transition times or some combination of them that causes the bugs)
-Mike (videoscribe user)
I'm attaching the scribe.
Note: if I render the scribe as a MOV video. the transparent images stay transparent...only when I render as PNG sequence does it add a white background to some of the transparent images.