Responsive menu is not working after update

Hello team I have just updated the theme. It really had an older version on it. But now the responsive menu for tablet and smartphone is not working. Could you have a look please. I have a child theme. so I think it could be in the additional css, which I made earlier. But I cannot find it. Other wise I have to set it back to the older version of your theme. that was working fine. the website is https://www.heilpraktikerschule-freiburg-gottenheim.de/ you see already that there is a little menu box left.... thanks Marga