Hi folks,
The solution to this particular problem can be found here:
http://forum.cockos.com/showthread.php?t=69700
Thank you both!
Yes, it seems that somehow the Quantize window was waaay off screen somewhere, so setting it's absolute position in the Reaper.ini file was the way to fix it.
No idea how the window got way off screen in the first place though.
Anyway - my day is now not in tatters.
Love
Thal