The larger the render resolution the more cycles it needs to complete the render. Let the cycles go till you are back to the UI in the render room. I think 3DC is breaking the render up into chucks, like a regular render does. We just do not get to see those little boxes moving around the render screen.That is my best guess.
5760 x 3054 took 9 or 10 cycles ( rays per frame set at 1000) to complete the render on my computer and save the file.
Hope this solves your render problem.