This is a very common problem.
Isolate the power to the appliance. (in case the lamp fitting begins turning and shorts the wiring out).
Use a silicon glove, or similar, the ones you get for opening jars.
It gives you better grip.
Grasp the lamp cover and rapidly, with all the strength you can muster, tighten and slacken the glass.
The small amount of movement will be the light fitting turning, the glass is still firmly stuck.
You do need to be quite brutal and use a good amount of force.
Hopefully, the grease/heat seal that has formed will give and the glass will unscrew.
If it doesn't, you'll need to obtain and try using the Bosch glass removal tool.
It comes with a spare glass, in case the old one breaks.
Click here for the tool and spare glass
It's marked Bosch; Neff, Siemens and Bosch are part of the same company.
If the glass on your machine doesn't resemble the glass in the picture, let me know.
Check it out, see how you get on and get back to me if you need more help
Hope this helps