Hi,
I had that same issue on my last 2 film. After 15 pictures taken, it becomes very stiff to the point I can't rewind.
I'm going to send it for repair but I have no idea what could be the cause.
Any idea ?
That could be the shutter problem. The gear could be broken. That's all I can guess. I had a similar problem with my M3 body before and had to get it fixed.