This one likely won’t be useful to many out there. It’s an issue I encountered with vCenter 5.x which manifested itself after the install of Windows Updates and caused a down time for my VMware Horizon environment.
We could see something was wrong due to the vCenter Server showing down in the VMware Horizon Admin Console. When checking on the vCenter service itself, it looked like the VMware VirtualCenter Server service was not started. When trying to manually start the service, it was getting stuck on starting the service for several minutes and failed. You may also see event errors with ID: 1000.
It turns out, a Windows Update enabled the built in Windows BranchCache feature. By going to the Non-Plug and Play Drivers in Device Manager and going into HTTP as seen above, we could see what was trying to use the HTTP protocol. (I was a little surprised as our environment uses HTTPS but it looks like vCenter still relies on port 80 when starting). Checking in here showed that BranchCache was butting in and taking over port 80 which was causing vCenter to fail to start. (Thank you VMware support for showing me!)
Simple. Disable BranchCache again. Chances are it was already disabled in your older vCenter before the update re-enabled it anyway.