I have had a similar problem.
I have two M645 Super bodies and one M645 Pro body. I also have three backs, and several inserts.
When I use one of my inserts, the dark slide interlock lets me release the shutter. If I move the back to a different body, I get the same result. If I try the same insert in a different back, I get the same result. If I switch inserts, the problem disappears.
I think it is the insert.