I believe that it may have something to do with the take spool/fork. There is a gear on top of the take up spool. It engages two other smaller gears that reset the shutter. If the take up spool is not on tight enough that gear can slip down a little bit and allow the shutter to slip. I believe that it is a matter of getting the take up spool tighter up against the top of the camera. I haven't worked on a Z4K in a while as I have been drawn to the earlier models. If you can watch the actions during advancement with the back off, that might make it easier to figure out.
Main Sidearm: Panasonic AJ-SPX800P
Backup Arsenal includes: Pentax 645, Nikon F3/MD4, Nikon F2, FED-3, Zorki 3 and a minolta Dimage A1