I am having the same problem. Brand new HP only had 2 weeks. Running windows 8. I updated to 8.1. Mail opened into 2 o3 windows, but then locked up. Also, other apps would not open from the Windows 8 screen. Uninstalled Modern Mix and then tried to download again to make sure had the newest version. Then it would not install kept saying Not responding. Also, had to uninstall Stardock too, because all was locking up. Everything fine now that they are uninstalled, but I miss using them. Had 8 on HP envy x2 and Stardock and ModerMix no problems.
What do you mean by you had to uninstall Stardock?
Stardock is the name of the company not any product. Do you mean Start8, ObjectDock, Stardock Central, or some other Stardock app?
Your initial issue sounds like you had an older version of ModernMix and so installing the update should have worked, but the not responding issue points to something else causing issues. I would recommend contacting support@stardock.com