problems with file attachment plugin version 5.8.1

Where can we see the website in question?

Here it works: https://sensescanner.de/hsp-und-scanner/

and here not: https://www.nivas-bielefeld.de/veranstaltung-einreichen/

Same Theme.

Thank you.

See Setting up mail

What email settings do you have in the Mail tab panel?

To protect personal data, hiding email addresses in the settings or replacing them with dummy addresses is strongly advised. You can also include screenshots.

It is not working.

Thank you for your help.

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

The Theme ist Uncode. But thats not the problem. The other site theme is also uncode.

Try switching to the default theme, deactivating all plugins excluding Contact Form 7, and clearing all cache data.

Why you are advised to deactivate plugins and switch to the default theme.