Consider the following scenario:
You sign in to Microsoft Lync 2013 in an environment that has a proxy server deployed.
You try to upload a PowerPoint presentation to a conference by using Lync 2013.
There are no external DNS resolution and external HTTP connectivity that are through the proxy server.
In this scenario, a proxy authentication dialog box appears unexpectedly.
Note The proxy authentication is not required, and you do not receive a 407 error.
If you input your credentials when prompted and then select the check box to save the credentials, the dialog box will disappear. The dialog box will disappear even if the credentials are invalid.
To resolve this issue, install the following update:
2825630 Description of the Lync 2013 update: September 20, 2013