location of [response], perhaps as popup?

Where can we see the website?

Sorry i forgot.
https://Www.hoegner-farben.de/wp/kontakt

What other plugins and theme do you use on the site?

I use avada theme and many plugins, to much to write it all down. Do you think on a specivic plugin?

Ok. Try deactivating all of them and switching theme to the default (Twenty Seventeen or Twenty Nineteen). Some of them are causing problems.