Recently we came across a problem where users would transfer attachments from one email to another and upon sending would get the following error

cannot open this item. changes made to the item were lost due to a reconnect with the server

The only way to send the email would be to recopying all the content to a new email and re-attach the attachments. It was though Outlook briefly lost connectivity with exchange server ( was not using cached mode ) . The connection status of Outlook showed no failed requests. We disabled all Add-on’s and started Outlook in safemode however we still got this happening randomly. In the end we applied all Office 2010 Windows updates to the terminal server and this fixed the issue!