So... thank you for all your replies, hints and tips!
I did a calibration, but it was indeed almost perfect calibrated - as I told, the problem was suddenly there.
But I'm a step further, I think.

I installed repetier host and when I'm using the Cura engine insted of the slic3r engine, the prints become better.
but they are not really good, the look is similar, but not as much as before.

And as I noticed before, there is like a click at the extruder, and in that clicking moment the filament isn't extruded. So this could be the cause of the lost filament inbetween.
I googled upon that clicking sound but I think there are different kinds of that click sound.
It is not a loud click, and in the beginning of a print there aren't such clicks. Most of the time.

Did someone have had those clicks ?

Greetings! Sebastian