I didn't change the operating system, but changes are constantly being made by others. It's alarming to be confronted with a programme, that has always worked from time of purchase (of the laptop) without any change in settings on my part, and which just suddenly stops working.
Let me emphasize that I use the programme to run 2d video clips in 3d - which is not quite the same as running 3d movies from dvd.
And it's 'until otherwise' 'coz I've only just stumbled on that as a workaround. It's an easy solution in my case and I'm sure ther's no harm in trying compatibilty mode - either it works or it doesn't.
Hopefully it will help others - there's another similar thread on wrong file formats which was the error message that kept popping up every time I tried to play a clip.