An update to this, I tracked it down to a group policy setting. I have a GPO that ONLY does one thing. It applies a proxy server to IE. That is the one and only thing it does. As soon as the proxy server is applied, start 8 stops working. It also seems to take about 5 extra seconds for the start8 icon to show up, almost like it is timing out when it cant check activation or something. Any help?

Solution to Start8 not starting after upgrading to Windows 8.1 Pro. What worked for me was the following. 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 helps.