thank you for your response. I´ve tried your advice. It seems the problem is in the plugin “styles-with-shortcodes” which is mentioned to be required for the theme. Since it was not possible to deactivate the plugin, I deleted it. Afterwards the visual editor was working again. But as soon as I but the plugin back in plugins-folder, the problem with the visual editor is re-appearing.
Do you have an advice for me how to solve this?
Thank you for your help!