If you copy pasted the MDS section from another, I can't recommend, for me converting was just successful by re-saving in FMB, that was my experience.
OK, Player was not selected in FMB checkbox to g0100, another thing, which is only formal, but for me very serious. If I'm Player, usually in 100% of the time and I'm Flightleader, most of the Time in QMB, I wanna start on the runway in the pole position quasi.
It is more difficult to make it like this, as the person who did it, manually switched the squadron. Maybe because he likes it if others start first. I'm arrogant, AI can wait. Better select side and create the Flights one after one, like in the Stock Missions always starting with Player and Correct side. Blue 0100 or Red 0100. FMB generates them to work out of the box.
After selecting Player I checked and loaded mission successfully.
These Errors are human, one forgets about the simplest task when in workflow.