It does appear to be a lock of some kind, have you tried restarting the server and testing it again? Perhaps migrate a new copy of the domain over, restart the server, then start SmarterMail to see if it successfully converts. Lastly; what are you using to migrate the data over? I've used rsync to move most migrated domains and haven't run into these issues.
Kyle Kerst
Acting IT Manager
SmarterTools Inc.
www.smartertools.com