- The computer that is running the Lync 2010 client does not join a domain that has Microsoft Lync Server 2010 deployed.
- The sign-in address and the user name are not the same. For example, the sign-in address is "firstname.lastname@example.org" and the user name is "DOMAIN\123."
- The Save my password check box is not selected.
- The update that is described in the Microsoft Knowledge Base (KB) article 2670498 is installed on the computer that is running the Lync 2010 client.