My guess for the shutter speed in the comment was based ob the approximate width of the bright strip relative to the image width, multiplied by the sync-speed. I assumed a 1/250sec sync speed.

Just a speculation: it looks like the shutter blades get stuck for an brief instance around the middle of the frame. It is possible that you have a piece of dirt stuck there, or that the spring or rails are about to fail. Try to decrease and increase the speed by a stop and see if the stripe gets twice as wide and as narrow. If so, it is indicative that it is indeed the shutter having a problem.