I went back an installed from a newly-downloaded full installer, no change.

Running the 32-bit version still works, though for some reason part of the Jazz chord font was missing. I changed to another font (BeStPlain) and that font seems OK.

I'd recently checked the 32-bit version and seen it was activated, which initially I took to mean that the activation from the 64-bit version had worked. I looked at the files in bb/Data/TicketsActivated and the probable activation is 11 December 2020 and suggests that the 32-bit version is happy with that activation on this same PC as the Win10 installation.

So rather rashly I deleted the bb/bbw*64.exe files and tried the 845 update again. Still no change.
There are some other x64 executables lower down the file tree, including in bb/Data/Backups/Builds, but the relevant .exe is identical to bbw64.exe.
I tried deleting:
Data/libx64/\$drums\^.exe
Data/libx64/\$midimon\^.exe
and reinstalling, but still no joy.

Time first for coffee.

Jazz relative beginner, starting at a much older age than was helpful.
AVL:MXE Linux; Windows 11
BIAB2024 Audiophile, a bunch of other software.
Kawai MP6, Ui24R, Focusrite Saffire Pro40 and Scarletts
.