A Microsoft Office Communicator 2007 R2 user creates a conference and invites an external user to join this conference.
The external user joins this conference.
The Office Communicator 2007 R2 user shares an application to the conference.
In this scenario, the external user is disconnected from the application sharing session.
This issue occurs because the Multipoint Conferencing Unit does not add the conference URI in the contact header. This behavior causes the session to expire. Therefore, the Multipoint Conferencing Unit disconnects the application sharing session with the external user.
To resolve this problem, install the following cumulative update:
2291724 Description of the cumulative update for Office Communications Server 2007 R2, Unified Communications Managed API 2.0 Core Redist 64-bit: November 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.