1. Un-hide the mailbox.
For more information review the documentation and the KB article 14667 (https://support.quest.com/SUPPORT/index?page=solution&id=SOL14667) which states:
* If a mailbox is hidden from the Global Address List, the Migration Manager agents will not synchronize this mailbox (including Calendar folder and free/busy message) and permissions granted to this mailbox.
2. Verify if DSA job is running and target accounts are mailbox enabled.
3. In case of the error code following the attempt to login into System Attendant mailbox like in the example below please verify that exchange server where the agent is installed is able to resolve the other side (source or target) exchange servers short DNS name. This is more common for Calendar Agent where it needs to login into both source and target mailboxes while installed only on one server.
01/06/2007 16:37:50 CSession::Logon TraceMsg 4800 Logging on to the mailbox /o=exch-server/ou=First Administrative Group/cn=Configuration/cn=Servers/cn=SERV/cn=Microsoft System Attendant (Server: SERV, user: target\qmm_svc).
01/06/2007 16:37:55 CSession::Logon Error -2147221231 The information store could not be opened. - MAPI_E_LOGON_FAILED (MAPI 1.0) Low level error: 0x0 File: aeWrapHelpers.h Line: 264