@danp Belatedly, many thanks

Host was running 8.1. Shut down all vm's and upgraded via booting from the 8.2 ISO. Now the main and crucial VM cannot start because it couldn't find the vhd. This has been happening for a few hours since the upgrade, interrupted by a 3h drive back to base. Now all of a sudden the VM starts fine. Anyone got any ideas what's going on?