Hi Steven,
thank you for your kind explanations. The theme for this website was made with Artisteer 7 years ago, so I’ll rather suggest the use of a new theme to the owner. Because the old theme will certainly offer one or two more surprises. ;-)
Thanks for your answer, I found the interfering plugin: WP Accessibilty.
It has an option called “deactivate direct links”. After disabling this option, everything works fine.
Thanks!