In Recognition Server 4.0, when connecting to SharePoint through Remote Admin Console (RAC), you may get this error message when trying to connect to SharePoint Online:
The underlying connection was closed: An unexpected error occurred on a send
One of the reasons such an error may occur is due to Microsoft .Net 4.5 is not being installed on the machine.
Make sure you have exactly .Net 4.5 installed on the machine.
Installing .Net 4.5.1 or .Net 4.5.2 will not work with Recognition Server.