Event handler - script missing

Edit (2) : Solved! It turns out that you cannot call static functions using the event handler.

Greetings, there seems to be a problem with finding an attached script to a GUI event handler.

As seen in the picture above, it cannot find the script GameHandler from Manager game object but it can handle WindowManager script just as fine. I was able to call functions from any other script without any problems but i cannot get this to work.

I have tried :

1) Moving the script to a different game object

2) Creating a new script with the same functions

3) Restart unity and PC and re-try steps 1 and 2.

no luck..

I would appreciate any tips.

Edit : This "Missing" problem only appears on custom functions.