The connection fails when running on a Windows Vista or higher operating system and making a DDE connection to the server from a DDE client or when connecting from the DDE Client Driver to a DDE server.

UAC is all about access privileges. The server is most likely running elevated with Administrator privileges and the client is running as a User. The DDE client must have the same privileges or higher as the DDE server in order to establish the connection. The connection should succeed if the client is running as an Administrator and the server is running as a User.

The client must be run as an Administrator in order to connect to the server. To do so, right-click on the shortcut to launch the client and then select Run as an administrator. If the client is launched at start-up, the shortcut properties can be edited to force the client to run as administrator.