Well, folks...

I checked everything you suggested, and saw no problems. I am, by the way, NOT using BIAB for any mixing, but rendering the tracks and then manually moving them to Mixcraft.
HOWEVER, I found the fault!!! In the Audio render window, the sample rate had changed from 44.1 to 48khz all on it's own. (Maybe due to the update?)
Mixcraft was set for 44.1, which I use for all BIAB work as I know that the P.A. will not deliver any remotely HIFI sound when I use the tracksw for backing!!!
So, many thanks to you all. You've given me new insight into the programme by making me look at new things.
All is now well, I'm happy to report!