I have a problem with BIAB randomly unable to generate one or more real tracks in a song.

The tracks are there, in my D drive where all my real tracks and drums are, and I can get the song to generate all tracks if I exit BIAB and restart.

Just another annoying bug I suppose, but as always I would be interested in suggestions for a remedy. This started on build 814 and continued with 822. I have the Ultrapak with all real tracks.