Thanks, Noel.
That made a bit of progress... now when I select Play -> Choose Loop Type, I get a quick flash of a dropdown menu which I think is asking how many bars to loop, but it quickly disappears before I get a chance to read it, then I get the usual message "Cannot focus a disabled or invisible window at:004636c7".

I have reinstalled the program (as Administrator) with update patch in C:\bb folder as recommended by the software but the problem remains.