Life Drain; what a pain!

I'm having trouble with an ability in my map that is based on life drain, which is slowly dragging the target unit towards the caster using SetUnitX/Y.

However, after a change in my map (Increasing hero max level from 10 to 18, one hell of a job) my life drain ability simply dont want to work anymore, it wont let me target anything.
My targets allowed is the same as base life drain.
I've tried to modify the targets allowed, and I've tried to create a new ability.

Anyone got an idea of why this is happening, and how it can be fixed?

I've had my share of pains with life drain. Sadly it won't allow you to cast it on certain things no matter what you change its targeted units to. I can't tell you exactly why (hard coded maybe), but it just doesn't work. Your best bet is just to make a dummy ability that creates a lightning effect (using triggers). from the 2 units and then dragging them.
EDIT: After reading your post again It looks like the problem only started happening after you increased the hero max level and nothing was really wrong with life drain earlier (while what I said earlier is still true if anyone else is having the problem i stated, but it doesn't really solve this). Maybe try setting life drain to default and then try editing it again. If it still doesn't work, then just make the triggered ability, it's still fairly easy to make.

Or try using Aerial Shackles (if you don't want the whole drain thing)

Check your gameplay constants for life drain behaviour flags. It can't target allies unless you specify it there, if that is your problem.