If this is the same bug than me, there is some undefined problem that makes the avionics non-functional. Typically, even with batteries OFF, the weather radar already displays some error message (in the Caravan, for example).
Only solution is to choose one of the default planes, and then kill + restart the sim.
After restart, try re-selecting the BlackSquare aircraft and launch a flight. It works for me.
But when the bug occurs, there’s nothing I can do to make the plane work. I have to kill the sim after switching to a default (or addon with no specific systems) aircraft.