I'd suggest there is an error somewhere else in your events causing the issue. I have just replicated your exact code and it works perfectly, although you have no redundancy for if both the player and box are overlapping the button, which will cause it to move twice. You could fix this by moving the door to var_doorYvalue+200 instead of self+200.
Also check that you have collisions enabled on your box sprite
Post your project file if not.