Microsoft released a KB article describing issues with the Web Components Server on Lync Server 2013 after installing the latest security update.
It affects the following:
•Users can’t sign in to your dial-in page.
•Lync Mobile clients can’t sign in.
•External clients can’t sign in.
•Address book web queries fail.
•Users are prompted for credentials for some web services after they sign in internally to Lync desktop clients.
To resolve this problem, uninstall security update 3080353, install the July 2015 cumulative update, and then reinstall security update 3080353.
Source and additional information: Microsoft.