Basically what the title suggests. I want a unit to be able to go down cliffs, but not up them. I tried editing the Reapers ability, but it ended up crashing the game when the unit tries to go up.

I don't see a way to do this by modifying the behaviour alone. But since it is a behaviour you may be able to do accomplish this by adding/removing the behaviour. (Maybe create an ability that jumps them up the cliff?)

Did you only do the Behavior? Some units don't even use behaviors (I believe the Colossus doesn't), and just use movers, and some use both (I believe Reapers use both). Might be something to look at.

I gave the unit the 'Cliff Jumper' Mover. I then looked at the Reapers behavior. I tried removing a few fields that relate to the word 'Up', but that makes the game crash when the unit tries to go up a cliff.

The jump behaviour seems to require you to go both ways which is why it crashes when you remove the up mover (it is still trying to use it). I would suggest maybe using triggers in this case to simulate a jump.