I've had this error twice now on mission 3. It happens after disengaging with the fighter ambush.
3 and 4 were there throughout the flight until the call to bomb the ground units. They always break left when 1 breaks right. I followed 3 and 4 in my last try and they seem to eventually circle back and find the ground units. Everyone from cobweb blue seems to be where they should be at the time of the fighter ambush. After this I turn north and eventually get the scripting error.
Maybe it's due to disengaging too quickly? If I stay and try to fight until 1 calls to disengage I always die, so I try to get out immediately.
I was able to finish the mission after closing the error dialog box so it's not game breaking.
Before flying it this last time, I did a slow repair of DCS and remove extra files. I also renamed my saved games DCS.openbeta file and let the game rebuild it in order to eliminate any mods or weird scripts.