Since it seems the mailbox move did not happen correctly, I just want to restore the old mailbox on the old server and cancel the operation, but that does not seem to be an option. zmmailboxmove offers --purgeOld, but what do you do when the move is not successful?
workaround: for some reason, we were able to get into the mailbox by using zmprov to manually set the account status to active... changes did not take effect immediately, but after about 10 minutes it seemed to work. There may have been other factors involved, but if so they did not appear in the logs. I would still like to know what the alternatives are for canceling a mailbox move request, though.