Well, I think you have checked all the obvious things, so we can rule those out.
Im really not sure that there is anything else you can do at this point, sorry.
Without a meter you are not able to test any of the internal components.
Sorry to say, I think you will need to call someone in.
I see you are in Germany, so I don't know of anyone there, but would suggest you ask friends and neighbours for a recommendation.
If I can help any further, please just ask me.
In the meantime,
Can I ask you to take a moment and rate my response? Please use the smiley faces which you should now see. Please remember you are rating me personally, not the website or the problems you have had with this machine.
If you need to ask me anything after rating, please just follow the link in the welcome email