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Hey badoes,
Thanks for your question, the file creating this error is a core WordPress file and not an Enfold file: /public_html/wp-includes/functions.php
the error line 5831 is a part of WordPress Debug notice for developers and your Sitediagnose recommends that you disable the WordPress Debug:

I also see that you are using an old version of Enfold: v4.7.6.4 with no Envato persoonlijk token, please enter a valid token and update to v5.1.1
If you have trouble updating your version of Enfold you may need to download the latest version from your Theme Forest account and upload it to your WordPress ▸ Appearance ▸ Themes ▸ Add Themes ▸ Add New

after you choose the zip file and click install, you will see a This theme is already installed message because you are updating, you can continue

then you will see the Theme updated successfully message.

After you update, you may also need to update your child theme footer.php as it has changed in the latest version and your old one will probably cause errors, I also see that your child theme has quite a lot of custom files.
I believe that the cause for the error is because Enfold v4.7.6.4 didn’t support the Gutenberg Widget interface at that time, so updating should solve.

Best regards,
Mike