This issue occurs when you join a meeting without using audio modality. Additionally, a user who uses audio modality in the meeting exits audio or leaves the meeting. When this issue occurs, you also see the Rejoin button in Microsoft Lync Web App.
Resolution
To resolve this issue, install the May 2015 cumulative update for Lync Server 2013 web components server.
More Information
This issue occurs because Lync Server 2013 does not check the original modality of users in the meeting when an audio/video user exits audio or leaves the meeting.
