0) I start by trying to set up a AWACS mission in the ATO. First i create 2 packages (awacs and escort) in the mission planner and set them to "flight planner only".
1) Then i went to the ATO, select the AWACS package there and select create flight.
2) In the ATO for the new package i selected "Change a/c type and loadout" and set the takeoff/landing and populated the AC with the AWE i assigned to the pool the package belonged to.
3) I edit the flight plan, as soon as i set the "task" to AEW i got the exception.

This seems to happen if you set the task before "creating flight".