(as you can see, I still have removed the most of the stain)
I tried to clean it with oven cleaner, with the green side of a sponge and I tried several cleaners without success.
the whole glass was like this brown spot.
I want to show you, how easy it is to clean the glass.

It is helpful to remove the door before u start.
in most cases this is easy, just half open the door and pull it away from the hinge.
if u have doubts, take a look into the manual of your oven.

Step 1: What You Need

-water
-a sponge or paper towels
-a dishwasher tab

Step 2: Cleaning

moisten the surface and rub the film with the tab. After that just rinse the glass.
You will see how easy and fast it is.
And the best, the glass doesn't get scratches.