That is a strange issue. My first guess would be that a content filter or proxy doesn't like something in the conference settings and returns an html error page as a response and the client is complaining that it cannot parse the data.One sure way to find out is to enable XML Logging on the client by following the steps below:1. Launch the Nefsis tray application.2. Log in to Nefsis.3. Click on the Nefsis application button ( green 'N' in the upper left corner).4. Click on the 'Settings' button at the bottom of the menu.5. Under 'General settings' check the box labeled 'XML Logging'6. Click on the 'OK' button.7. Log out and Exit Nefsis.8. Restart Nefsis and try scheduling a conference.Assuming the client crashes, run the nefsisDiagInfo.exe attached to this post (look at the top of my post for the download link). This small application will obtain some Nefsis and basic Windows diagnostic information that we can use to troubleshoot the issue. The files will be saved to a folder on your Windows desktop. Simply email these files to firstname.lastname@example.org and we'll review them as soon as we have them in hand.