Hi @Wiebke - welcome to our community! :)
I think I know what has happened here. It was previously necessary to use some CSS in order to have more than 3 quick links in a row. A few weeks ago, however, we updated the quick links widget to natively support 4 or more links per row. That change, however, in some cases doesn’t play nicely with some of the custom CSS that some of our customers had been using.
To remedy this, I suggest you quickly go into your CSS and ‘comment-out’ the lines that are related to quick links layout. I had a quick peek in your community and they’re thankfully nicely described and easy to find. I’m fairly sure that will fix it for you.