Primary menu not in columns on mobile

Hi! I am having trouble with the menu of my site in mobile view. Unfortunately, the items are now shown below one another in an orderly column (as for instance on the ThemeIsle site) but next to one another. Also, the drop-down arrows are overlying the text and when opening the drop-downs thesre are even overlaying the other menu items.
You can view pictures of my site and of the ThemeIsle site under the link below so that you can see what I mean.
Any help is greatly appreciated! Thanks!

https://postimg.cc/gallery/26L2prw

• This topic was modified 2 years, 3 months ago by Jan Dembowski. Reason: Moved to Fixing WordPress, this is not an Everything else WordPress topic

The page I need help with: [log in to see the link]