@Kuba thanks for testing on your side. It turns out that MessageDialog["test"] stopped working because I have a new monitor!!! Arghh!!
With my new amazing 4K 32" monitor I can set the Windows magnification to 100, 125, 150, 175, 200, 225, 250, 300 or 350%. When I select 125% then MessageDialog["test"] fails. It works perfectly for any other of the magnifications on the list. Go figure.