Thanks for the advice, chrissmit and esmi.
However, I found out that I wasn’t running the latest version of my theme. I updated my theme and suddenly everything was up and running like it should. I’m quite sceptic, whether I should leave it as it is, but since I don’t have that much content on my site, it could be an option (I have backed up my site). I changed my admin password and I’m about to change my FTP’s passwords too. We’ll see.