RB67 Pro S Cocking lever locking

[JIMM]

I have recently acquired an RB67 Pro S in well used but serviceable condition. It came with a “C” 127mm lens. Both 127mm lens and camera work normally. But I was also given an older non “C” 90mm lens, when this lens is cocked and mounted to the body the shutter fires normally, but when the camera is re-cocked the cocking lever locks at about mid travel and cannot be moved, and the lens now cannot be unlocked in the normal fashion. I can remove the lens with a “paperclip” through the hole in the camera front.
The operation of the lens shutter appears normal, the shutter cocks and fires normally with no sign of damage; can anybody suggest what the problem might be?