I finally solved it!
It was a dodgy plugin/extension. I don't know which one, but luckily it wasn't one of the ones I use constantly. I probly had 50 old plugins from years ago that I've never used.
This only became clear when the Space Mouse in the trial version of SU24 stopped working after I had loaded a few Bundles of Extensions. So I did a clean re-install of SU24 and loaded my necessary plugins one by one. I stopped when I had everything I needed and all was good. Then back to SU19, I used the Extension Manager in SU to uninstall everything except the proven ones in SU24 and bingo, all is as it should be.
Note: Rich's suggestion of a 'Vanilla" install, (see above), did not work in my case, though I don't see why it wouldn't.
Cheers and Happy New Year to you all.