By default, the On Demand Migration for Active Directory Agent service is running the context of
Local System account. So please check the SMB Share Access level and ensure that the local system has the appropriate access to the share. If the agent service is running with a different account, ensure that account has appropriate access to the share.
For more information on the local system, please refer to
Special identity groups . As described there, the local system must be accessed with ANONYMOUS privileges.