Here's the video:

https://www.dropbox.com/s/yl8g9u983td4c ... 6.mov?dl=0

I should add that the play/preview function works fine, and that it only displays that error after reaching 100% on the overlay progress.

Trying to overlay on a pip video I made in Final Cut Pro, with video from the iPhone front camera on top of footage from a 720p Drift Innovations HD170. Error reported is '-11800: The operation could not be completed'. The video was tried in various different formats, including H.264/AAC in 720p 60 ...