maddy, philebus:

While you're waiting for EF to address this bug, you can still create lengthy animations with the Sketch Renderer ... you just can't render more than 40 frames at a time.

Set the movie output to image sequence. Render frames 1-30. Then render frames 31-60. Then render frames 61-90. Repeat as necessary until all your frames are rendered. Compile the images into a video using a third-party utility like Animation Shop or VirtualDub.