You have a number of virtual machines running on your Windows Server 2016, Windows 10 or Windows Server 2008 R2 Hyper-V system. As soon as you try to start another VM in the Hyper-V Manager console or Start-VM virtmachine via PowerShell, a rather cryptic message keeps preventing you from doing so:
Virtual Machine Connection
The application encountered an error while attempting to change the state of ‘virtmachine’.
‘virtmachine’ failed to change state.
The operation cannot be performed while the object is in use.
To resolve this, disassociate any other VM from “Physical CD/DVD device” under media before you retry the operation on the failed VM.