An Instant Messaging conference is hosted on a Microsoft Office Communications Server 2007 R2 server. When a user sends a message in the conference, another user who is in the conference is disconnected from the conference. In this case, the message sender does not receive any notification that the message is not delivered. This problem occurs because of an Office Communications Server load issue or because of a local network connectivity issue.
To resolve this problem, install the following update:
978008 Description of cumulative update for Office Communications Server 2007 R2: April, 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.