Which version of RB67 - "Pro", "Pro-S" or "Pro-SD"?
The different film backs + rotating adapters have different safety interconnects. And if I recall correctly, the polaroid back doesn't make use of the rotating adapter.
It may be that there is a problem with the interconnects and/or the rotating adapter, such that the shutters on the lenses aren't releasing when they should.
Try loading the backing paper into the film back, advance it normally, and then watch the leaf shutter as you release the shutter release.
By the way, if you load it properly you will see the arrows that show you where to stop the film. My question would be - what did you see when you opened the back at the end? Was the "exposed" sticker there?