Also getting this "An unsupported Operation was attempted" dialog box popping up every time I start PM 4.6. I suspect some users are reporting this as a "hang on start" situation, since the dialog box is UNDER the splash screen. Since the splash is transparent, you can see the dialog if you look closely; and you can alt-tab to bring it to the top and click it, at which point it goes away and everything seems to be fine.
Windows XP SP2 here as well.