Assume that you deploy a coexistent on-premises Microsoft Lync Server 2010 server and Office Communications Server 2007 R2 Edge server. A user tries to sign in to the Lync 2010 server from an external network by using Lync Windows Store App. In this situation, the user cannot sign in.
To resolve this issue, install the following cumulative update:
2847898 Description of the cumulative update for Lync Server 2010, Core Components: July 2013