Video rendering error -50

I'm trying to 'share' a 8 minute movie I made with imovie 2013, but after ~10 minutes and a lot of fan noise, I get a movie only about half as long as it should be and a "video rendering error: -50" message.

The movide is a bunch of pictures with transitions, some text overlay, and audio throughout.

If I continue to let it run, every ~10 to 15 minutes I get a "Share Failed The share operation "xxx" has failed." message, but the fan is still making a lot of noise and activity monitor shows imovie consuming a lot of CPU. The size of the file on my desktop (where it is supposed to share to) has not changed.

Does anyone know what a "-50" error is?

I'm trying to do this on a Late 2012 13" MBPr with an i7, 8GB RAM, and 500GB drive (112GB free).