The virtual machine cannot be used because its files are corrupted. I had made a backup of this VM and that I get the same message when trying to start that VM. The .hdd file seems to be ok, because I can double-click to mount and explore it. Is there something I can look for to manually edit in the .pvs file that may help recover this?
Well if the .hdd is ok you might want to move it from the corrupt VM and set up a new VM (take care you use the same MAC address as the old VM - you'll find that in the .pvs file) and use the old .hdd file in the new VM. That may be easier than trying to figure out just what is wrong in the old .pvs file.