The file could not be exported because of an internal error

I get this error off and on when I try to “share” a slide show as an .m4v.

The file “xxx” could not be exported because of an internal error

It goes through the entire process of copying, etc, and when it seems to be done, it breaks off.
Last time I found I could saw a low-res version instead of a 700p version, later the export worked,
but the next time it balked again. This is frustrating as I put a lot of work into this. What can I do?
I’m on vn 3.8.1 - I’ll try 3.8.3 (Foto Magico said it couldn’t unpack the update, so I guess I’ll have to reinstall entirely) but I don’t see problems in export mentioned in the release notes.