The service account used by VMAN needs to have permissions to perform the operation. In our environment, the account is assigned Administrator permissions, but you might be able to fine-tune it. This is the account defined in the Setup tab in the VMAN interface under Credentials which is used to access vCenter.
Tom is correct. Can you perform a migration using the account used to access the vCenter?
Can you provide the ticket number so I can escalate the case and track it?