Q: Apple Compressor "Error trying to open source media file." XAVC-S .mp4 files

Whenever I try to open Sony A7s XAVC-s format .mp4 files using Apple Compressor, I get this error: "Error trying to open source media file."

Does anyone know how to get around this or what it means? I am trying to batch convert a few hundred files into a lower quality format to send to an editor in another part of the country.

This happens whether I import a bunch of files or just one. There has been the odd one which did open in Compressor, but this was rare and by accident, and I could never duplicate it.

All these files open fine in Final Cut Pro X and the Adobe suite of products. It also happens on both my iMac and Macbook. I have tried reinstalling Compressor from the app store, no luck so far.

Thanks for any help.