Symptoms
This issue occurs in a Microsoft Lync Server 2010 or a Microsoft Lync Server 2013 environment that has multiple front-end servers deployed. This issue occurs because multiple Audio Conferencing Provider (ACP) Media Control Unit (MCU) services send the INVITE and the SUBSCRIBE requests to the audio conferencing provider module for the same meeting.
Resolution
To resolve this issue, install the following update:
2881070 August 2014 update for Lync 2013 (KB2881070)
