The following event is logged in the Application log on a Microsoft Lync Server 2010, Web Conferencing server:
This issue occurs because there is a certain time frame that the Archiving server does not process messages. Therefore, Data Multipoint Control Unit (MCU) receives acknowledgements (ACKs) that do not match ACKs from Message Queuing.
To resolve this issue, install the following cumulative update:
2530592 Description of the cumulative update for Lync Server 2010, Web Conferencing server: April 2011
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.