If memory serves, Repetier Server only talks to Repetier firmware, which isn’t on your Rambo. You want to uninstall Repetier Server, and install Repetier Host.
edit: Repetier Server also monopolizes your USB port, and won’t allow anything else to connect to it (Pronterface, CNC.js, etc., etc.)
edit2: Or I could be wrong, and you should listen to Jeffe, who is much more knowledgeable about these things. 