Same issue here with Windows 8.1 x64 and KB3000850 (Windows 8.1 Update R2 / Windows 8.1 R4)
KB3000850 not Update 3
KB3000850 Replace KB2919355 = Windows 8.1 Update R2
winblue_r4.141028-1500 = Windows 8.1(Blue) R4
What I did:
1.Avast 2208 (Only File system shield, Web Shield)
Disable File system shield, Web Shield, Self Defense Module, HW virtualization
2.Update KB3000850
3.Restart & Login
4.Same problem here
5.Open Task Manager
Runonce.exe /Run6432 causing this issue?
6.Uninstall Avast in safemode
7.Restart & Login
8.Open Task Manager
Runonce.exe /Run6432 don't exist