There's nothing wrong with your camera.
Your camera is trying to find something on which it can lock auto focus. Since the set is dark, there are no lines for it to resolve. As you said, a modeling light adds enough light for the AF to find a subject. You can also set focus and turn AF off if you're using a tripod and your subject won't move.
If you were using the popup flash, the AF assist lamp would fire to give the AF some help. Since you're in manual mode (and the flash is down), the camera cannot fire the assist lamp.