Thank you!
Ah, someone noticed it xD noooooo
It happend because of the post-process compositing of darkening the glare on the 'inside' of the eclipse: it darkened the very top of the mountain, too. I could've worked on it more aka differently, figuring out how to properly use different render layers, but I had personally not much more time left so I just had to run with it .-.
(I thought no one would notice ^^")