Hi Biagio Aeberhard,
If i read you right then this issue sounds like a potential bug however just test it more to be sure that there is no typo while entering the passes and not passed the time validity and device limit.
Do you see this issue with older firmware and with ZD generated guest passes?
some of the guest passes which stopped working, what error they throw?
If all has been done and verified, issue still exist then time is ripe to open a support ticket with problematic guest passes and ZD debug file.
Best of luck.