Yes, I did rollback to an earlier Restore Point to when i KNEW BiaB was okay---to no avail. Anyway, by jove, I think I found it. Check this out:

The problem seems to occur when the “Mandolin, Rhythm Bluegrass Doc EV 100, #599” RealTrack is used, which is the case in the style I was working with. I selected various other RealTracks and my problem disappeared. Went back and tried “#599” and had the same problems again. BTW, I noticed that in the “Instrument” column in “Assign RealTracks to Track” that EVERY Mandolin RealTrack has “<No Patch Change> (0)”. That may be significant to someone in solving what I believe to be a glitch in the #599 RealTrack, although there seems to not be a problem with one of the other Mandolin RealTracks I chose. Incidentally, I have not tried all RealTracks yet.

I still have NO IDEA why once the error occured the file would no longer work even with other various style choices. I can only guess that the file had somehow become corrupt after receiving the error.

At any rate, you have all been so helpful, and the upside here is that I have a wonderfully clean, functional machine. LOL

Thanks so much!