Single/multiplayer missions

[slm]

If I create a mission that is playable as single player, should it be possible to play the same mission as multiplayer also?

I made a simple mission that seems to work fine as single player, but when I create a MP server and try to play the same mission, several planes explode in air. Those planes start from altitude of 2500m, yet error message says they collided with landscape.

Any ideas on what I need to change so the mission can be played as MP?