Lync 2010 continues to sign out and then sign in every 30 minutes

This occurs because Lync 2010 does not clean the old Security Association (SA). When Lync 2010 tries to query the presence status of multiple contacts who are in corrupted state, it receives a 480 error, and then loses the connection to the front-end server. After reconnected to the server, Lync 2010 tries to resend the subscription every 30 minutes. When it resends the subscription, Lync 2010 creates a new connection that has the old SA or an empty SA. Therefore, the disconnection and then reconnection occurs every 30 minutes.