Hello!
Thank you for your reply!

I got it working by just trying around.
The Problem was related to insufficient filament flow. When i increased the Extruder-Temperature, the print got much better!

I am now printing PLA with 220°C . It is quite high, but works very well.
I dod not change Filament flow.

For me, the printer works very well. I am quite impressed by now.
Hopefully you can figure out your problems, too :-)

PS: I will try, if chooying another filament size makes it even better. 1.76 was, indeed, the measured size, but just because i am curious, i will give it a try!