Hyper-V guests all-of-a-sudden cannot find their disk and hence cannot power on

All of a sudden - Hyper-V guests all of a sudden cannot find their disks and cannot start:

An error occurred while attempting to start the selected virtual
machine(s).
'SERVE01' failed to start.
Synthetic SCSI Controller (Instance ID
C2570DAD-C5CA-4AD9-9E12-2375ADD176D6): Failed to Power on with Error 'The
system cannot find the file specified.".
Attachment 'G: \Hyper-WHard Disk
Files\SERVER01 _5FA5DA32-7007-4F18-B7A9-8B21F8F38C27.avhdx' could not be
found due to error: 'The system cannot find the file specified."

I checked for the disks and they are in that same folder. I read somewhere that there might be a rights issue in an XML file or even on the disks themselves all-of-a-suden? This host with 3 servers on it has been running fine for over a year no issues, all of sudden on a December Sunday night of all nights all 3 guests cannot find their disks and they all give a similar message as above for why they cannot start!