Yoast SEO meta box is blank
If the Yoast SEO meta box looks like this on your posts, pages or custom post types, you know there's something wrong.
Avast is blocking Yoast SEO
The antivirus software 'Avast Antivirus' has a feature called 'Script scanning'. It is supposed to block malicious scripts, but has been reported to block important Yoast SEO files as well.
To prevent this feature from blocking Yoast SEO, you can add your own URL to the 'Script exclusions'. For more information, please refer to Avast's FAQ at avast.com/faq.php.