I had vaguely similar issues with my Certo 6.
I looked everywhere - inside the film chamber, checked the top plate, checked the bellows, checked the lens and shutter, tried the camera in and out of the case, & even tried renewing what looked like some non-original felt around parts of the door.
Then I went and looked at all my other folders, and noted that none had any light seal material, and it twigged as to why the back was always a little fiddly to open - I surmised that there should be NO light trap material, and that it is entirely "mechanical". So I removed all traces of light trap material, and the problem has gone away.