I had a few problems creating videos awhile back.
A couple of things got me back on track.
The first to try is, after the video is created, be sure to close BIAB before trying to play the video.
If that doesn't cure things,
the second thing to try is, after hitting the "Save as video ..." use Ctrl+T to put BIAB into "condensed" display,
THEN hit the OK- Render Video button.

If all that doesn't help, try reading through this thread to see if there's anything else you can try.

https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=431945&page=2

I was equally frustrated at first, but once you get it working, it's a pleasure to use.
Good luck!
LLOYD S