I will now give the solution of the problem for myself.
Hope so somebody will save a lot of time.
When you start Photo Mechanic 5, right click it's icon and then select the Run as Administrator option from the contextual menu. This will let Photo Mechanic write its registration files in the ProgramData folder.
If Photo Mechanic will not open a blank contact sheet, then I would reset its preference files. Here is the path to Photo Mechanic 5's preferences :
C: -> Users -> AppData -> Roaming -> Camera Bits, Inc -> Photo Mechanic
If you rename or remove the Photo Mechanic folder, Photo Mechanic will create a new preference folder the next time it starts up.
If Photo Mechanic opens with the new preference folder, then go to the Edit menu and select Preferences. Go to the Caching page and increase the Memory Cache size from 256 MB to 800 MB. Ok the preferences and restart Photo Mechanic.
I hope this gets Photo Mechanic working correctly on your W8.1 PC.