Most likely, you have an older type ceiling outlet box that was supported by a fixture hickey in the middle.
This type of box is not suitable for ceiling fan support.
Depending on how that box is fastened to the framing of the ceiling joists, it could be a simple matter of using extra long screws to fasten the fan bracket to some wood structure above the box. Just install the screws beyond the box's edge and up into the wood above.
If there is no wood structure above the box, you may have a situation where the hickey mount was attached to a thin metal bar stretched between 2 joists. If that is the case, you may have to tear that out, and use an expanding fan support box/system in its place.