The most likely cause is a sticking shutter.

A shutter that fails to open properly can produce frames that are completely black, and the fact that the first shot after power-on or a mode change often works fits that symptom. Since you’ve already ruled out lenses, cards, file format, and firmware, the camera body is the likely source.

A simple check is to remove the lens and see whether the shutter appears stuck closed when it shouldn’t be. If it is intermittently failing to open, that would explain the black frames.

This is generally a hardware fault rather than a settings issue, so there usually isn’t a menu fix for it. If the shutter is sticking, the practical solution is camera repair/service; otherwise replacement may be more realistic depending on repair availability for the NX system.