I experienced this error recently and spent an eternity trying to resolve it. I finally found the solution to my problem and wanted to document it for others.
For me, the issue only appeared when I tried to invite people to a scheduled meeting. Under normal circumstances, blank (no invitees) scheduled meetings worked fine. Ad-hoc meetings worked fine. Some invitees worked, but others did not. When an invitee caused the issue to appear, this message would be displayed either when I saved/sent the meeting, or tried to set an invitee as a meeting leader.
It turned out to be caused by the format of the invitee information. Typically, on an Exchange-based email network, your global address book contains all the people within your company. Sometimes, emails will get sent around that results in employee names being displayed something like My Name <my.name@company.com> instead of just My Name. These entries can get cached in a frequent recipient list that will pop down when entering addressees.
Well, it turns out this format causes the Conferencing Add-In to barf. When entering recipients into a LiveMeeting request, entries in this format (as opposed to the usual Exchange My Name format) will cause something somewhere to fail, resulting in this message. Keep in mind that the name will look correct in the To: field, even though “behind the scenes” it’s actually the lengthy format above.
To resolve, either select a user from the Global Address Book when entering attendees or remove the entry for the cached list by hitting Ctrl-Delete while the name is highlighted in the list. Then enter a name, force a lookup on the user (“Check Names”), and all should be well.