3.0.5.5992 dialog boxes don't appear

Win64

When first opening MuseScore 3, the opening dialog doesn't appear - I have to hit escape to get control back. But I can't do much useful as any(?) action which pops up a dialog has the same effect - no dialog appears but it must be "somewhere" as the only way to get back control is to hit escape.

For example, if I right click on the first bar and select "Bar Properties...".

This is unfortunately a bit of a showstopper.

I've tried uninstalling/reinstalling and both 64bit and 32bit versions - no help.