When you sign in to Microsoft Lync 2013, a proxy authentication dialog box appears unexpectedly. This issue usually occurs when Lync 2013 tries to connect to a proxy server to download an external picture for an external contact. This issue may also occur when the external network has a proxy server even though the local network doesn’t have a proxy server.
If you input proxy authentication credentials, the pop-up dialog box disappears. However, if you don’t select the Save the credentials check box in the dialog box, this issue will occur the next time that you sign in to Lync 2013.
Generally, your Windows credentials authenticate you to any proxy servers automatically.
To resolve this issue, install the following update:
2825630 Description of the Lync 2013 update: September 20, 2013