Any time I was exporting movies from Final Cut Pro X to Compressor through the Share –> Send to Compressor option to render them within this application, Compressor would fail randomly during the process with a QuickTime Error -50.
I couldn’t really make sense of the error as it happened randomly, meaning sometime the rendering would be successful, sometime not. It also appeared to be independent from the encoding parameters utilized. Tweaking with them did not help at all.
Looking up on the Internet I found many others with the same problem, but none of the recommended solutions worked for me, so I had to look for my own!
Long story short, here is what worked for me. Are you ready?
Well the solution is: do not close Final Cut Pro X after having issued the Send to Compressor command and switched to Compressor!
Apparently Compressor needs Final Cut Pro X still up and running in the background in order to be able to render successfully. Folks with not enough RAM/CPU power may think to free up some memory by closing Final Cut Pro X once the Send the Compressor command has been issued (it was so for me, before I upgraded my Mac Book Pro to 8 GB of RAM).
Closing Final Cut Pro X while Compressor still has to render/encode the movie causes it to fail with the cryptic QuickTime Error -50. Since I have been leaving FCPX open in the background, Compressor never failed once!
Hope this helps!