Don't you simply have a very small light leak in your bellows? Since the Efke film needs to be rated at very low ISO, say 1-6, you will always have long exposure times, and hence probably also considerably longer times with the dark slide open compared to "normal" film shots. You may therefor not have noticed the fogging with normal film... Have you tried leaving a normal piece of film for some time in the camera with the dark slide open and see if it gets fogged?

Or better, put some LED light in the camera and take the camera into a completely dark room and see if there are leaks anywhere.