Yup, start at the bottom first. I suspect the side opp the lever though, have seen lots of bent linkage problems with symptoms like this.
Although he says it started out of the blue on the last frame.
I once had an RB that mysteriously had a nut floating around in it that jammed up the works, maybe got in there from the mirror drive slot inside the camera when stored in his case without a lens or back?
Weird things happen.