If I succeed in an Incapacitate ranged attack and I am also using Enhancement, I assign the target an action token and deal him damage. The same effect can be obtained with a successful attack using the Light Post 3D object.

What happens if the target has already an action token and he has the Toughness power only on his current click?

Since the target has Toughness, he would reduce the damage dealt by the Enhancement or by the object. But if Incapacitate takes effect before the damage is dealt, the target is dealt pushing damage, he loses his Toughness and the damage dealt by the Enhancement or by the object would not be reduced.

Which would be the correct timing in such a situation?

I'm 99% sure push damage is resolved after everything else. I can't quote the exact spot in the rulebook nut otherwise you'd have to worry about taking damage in the middle of an attack you pushed to do.

Tarnish

Can't quote it but I know that the timing will go... damage, token, push. See if you roll a crit hit then you hit someone with the extra 1 damage (as it said do zero so this is one more) and that will be Enhanced (so 2) now they have taken 2 damage and now much recieve a token and get their push... and finally get the knockback and possible damage there in.

trust me I made a Psylocke/Nightwing/Ex. Taskmaster/Green Arrow team and made certain of the rulings for my Enhanced double Incap team.

Supes is correct. Pushing damage is taken into effect after the rest of the action is completely resolved.