Had a similar problem i.e, Start8 not starting after upgrading to Windows 8.1 Pro. The following is what worked for me. I went to the Start8tweak.exe folder in C:\ProgramFiles\Stardock\Start8. After clicking on that folder, the following window Start8 Advanced Tweaks appeared. I then placed a check mark against “Start8 should be enabled in this user account” and my Start8 application started. Hope this solution helps.