Thanks for your reply, allthough I must apology, it was all my fault. I had the same text on all the button and thus could distinguish them. I had only put my OnClick script on two of the buttons and thought they were the first two. But the order was not the same in the ScrollView as in the hierarchy (of course, duh!). It was working all along.
Now though, I noticed another problem. Only some of the buttons work, the rest are not even playing a sound. When trying to find a difference between them I noticed that under the "Box Collider" the Center.Z keeps getting a lower number for each button I add. When changing the Z value to an equally low number as a working button, it works.
So the "Box Collider" is never hit, but seems to be behind something.
I reduced the Transform.Z for the UIGrid object to -2 and that solved it. Is this the correct way to fix this or am I missing something?