Categories not working

Thank you for a great theme. I’ve downloaded it a couple of days ago and am experiencing some problems.
The categories are not working for me. No matter if I click on them in the menu bar or below the posts – I only get an error page and don’t know what to do about it. Is there an easy fix for this?
The header image is weird as well – it seems to be repeated though it should have the proper size.
Below the posts you also see an arrow with the text “%title” next to it which is probably not how it is supposed to appear. Would you help me out on this? Thank you

https://www.dietesterin.de