Another guess - can you try installing Converter server on another machine and run reconfig from there?
Rationale: converter server (actually worker) uses a driver to mount the destination VM's virtual disks. This is used by reconfiguration which make some changes in registry as well as some file operations.
This workaround might help In case there is some special software (driver?) on the machine where converter runs that interacts with converter's driver and prevents opening the VM's registry.