Hi Wicky,
clickable elements are too close together
For sure add bottom margin for buttons inside block "Schneller Kontakt" , right now buttons are too close!
und
use Accessibility tool to find source of problems , for Firefox you should have "WAVE" add-on.
text too small to read
On first look, hard to say where to look exactly. But indeed in some areas font-size is smaller than in others. For example here "Wir rufen Sie gerne zurück. Hinterlassen Sie uns einfach Name..." there is only font-size:12px (too small as we can say!)