I upgraded from 1803 to 1809 last week, before Microsoft withdrew 1809. Everything was fine as far as I could tell. Until today. Today I opened Hyper-V Manager so I could start my Windows 8.1 VM. When I open the Manager, though, it doesn't seem to think I have Hyper-V running on my system. All I have in the console is the option to connect to a server. When I try to connect to my local computer, I get the error "An error occurred while attempting to connect to server "xxxx". Check that the Virtual Machine Management service is running and that you are authorized to connect to the server." The service is running and I'm the admin on this server so I guess I'm authorized. It worked up until this upgrade.
Thanks for this suggestion - it's the first solid useful suggestion for why Hyper-V has been broken for me since last November or thereabouts. I'll be directing people here in future. I found that I also had to disable Control Flow Guard protection on the 'vmwp.exe' process. Once I had done this, I simply restarted, and Hyper-V works again. Irritated that I didn't find your solution last year!