In my case at least the issue seems to be registration/authentication of the Windows store and the apps. I am resolving the issue. First I had to re-establish the ability to run the Windows store app, which I was able to do after I found a shell command to re-register it (see http://answers.microsoft.
I am still having the same issues after unistalling Start8 and ModernMix. So it appears to be a Windows install problem not a Stardock problem.
I am going to temporarily uninstall your products as a troubleshooting step.
Haven't tried them all but of the ones I've tried I can report the following results. Crash at start: Mail, People, Calendar, Weather, News, Finance Open but hang at startup screen: Twitter, Music Seem to Work properly: SkyDrive, Photos, Health and Fitness, Food and Drink
Same issue. I'm running ModernMix 1.12 and most Modern UI apps either just stall fullscreen or just crash. The one exception is the Skydrive app; it seems to work properly. One other thing in case it matters is that I'm also running Start8 most current version as of this date.