I was creating a new virtual machine, and was getting the error “0x80070003”, “the vm configuration […] is no longer accessible”.
After some research and experimenting, I found a working solution:
Open the Hyper-V-Manager, right click on the PC and then on “Stop Service”.
Delete “%systemdrive%\ProgramData\Microsoft\Windows\Hyper-V\Virtual Machines Cache”.
Go back to the Hyper-V-Manager, right click on the PC and then on “Start Service”.
Now create a new virtual machine, and the error should be gone.