Camera not connected

What causes "Camera not connected " on the Litchi screen ?

Black screen on LItchi but screen working fine on DJI go4.
Spent two Mavic batteries struggling to get this working.

Sat in a remote field 12 miles from home, I ended up downloading
Litchi again on a crap internet connection on the Ipad.

This fixed the " Camera not connected " problem, but then I had
only half of the third battery left, so I had to give up and return home.

Force Closing every app and restarting Litchi or restarting the RC would most likely have solved it.