The maximum number of connections allowed by the user has been reached.
In Exchange Server 2010, the number of connections are now limited per user. Out of the box, the default behavior is that each user is allowed to open ten (10) connections concurrently.
You are most likely to hit this limit when performing a migration while using administrative credentials. The single user account used will open one connection for each mailbox being migrated. If you are migrating ten (10) mailboxes simultaneously, you will want to increase this connection concurrency limit.