Sudden PHP Error

Hello, I have been using this plugin for a long time and have been very happy with it. So that I don’t want to change it.

Now we suddenly have a PHP Error on both pages with forms:
https://www.tanz-birke.de/anmeldung/
https://www.tanz-birke.de/kontakt/

The form is still working though.

The Error reads like this: Warning: Invalid argument supplied for foreach() in /homepages/30/d17491080/htdocs/tanz_birke_neu/imida-webdesign/wp/wp-content/plugins/visual-form-builder/public/class-form-display.php on line 507

We are not aware of any changes that could have caused this error.
Could you advise? Any help will be much appreciated.

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