I have a Simrad AP 44 and an AP 48 on flybridge, connected to an AC42. Chartplotters are Furuno, TZT9 and 14.
Occasionally, after entering a waypoint or "Go To", when I try to Nav, the AP will not allow me to select "NAV". Often, if I turn off all electronics, and re-boot, it then will allow me to select "NAV". Sometimes, it will not. A day later, usually just fine.
This is very frustrating as it is very un-predictable.