Although you enter your Microsoft Exchange credentials when you are prompted in Microsoft Lync 2013, the prompt continues to appear.
Note This issue may occur every one or two months.
To resolve this issue, install the following updates:
2850074 Update 2850074 for Lync 2013: June 2014
2881013 MS14-036: Description of the security update for Microsoft Lync 2013: June 10, 2014
Note After you install the update, the dialog box that prompts you for your Exchange credential will not appear unless you click a notification bar in Lync 2013 or a small icon at the bottom of your computer screen.