Lock-Up on Z3
I have the same problem. I did upgrade the firmware, but that has had little, if any, effect in solving the problem. I haven't been able to prove it is due to battery strength either.
So far, the only solution is to manually shut it down and then turn it back on (re-boot ). What is interesting is that, if I can get it to auto-focus in a low-light situation on a particular subject or scene, it will continue to work. The "lock-up" problem, when present, occurs on the first auto-focus attempt.
I don't remember if it happens in manual focus.