Error message: Failed to set referrer policy: The value '' is not one of 'no-referrer', 'no-referrer-when-downgrade', 'origin', 'origin-when-cross-origin', 'same-origin', 'strict-origin', 'strict-origin-when-cross-origin', or 'unsafe-url'. The referrer policy has been left unchanged.
Symptom (in Chrome): load page and check console
Error message: Failed to set referrer policy: The value '' is not one of 'no-referrer', 'no-referrer-when-downgrade', 'origin', 'origin-when-cross-origin', 'same-origin', 'strict-origin', 'strict-origin-when-cross-origin', or 'unsafe-url'. The referrer policy has been left unchanged.
Setting in WP: https://www.mmmnmnm.com/wp-admin/admin.php?page=w3tc_browsercache
Hint: flag may be unset (https://wordpress.org/support/topic/failed-to-set-referrer-policy-error/)
Background: https://scotthelme.co.uk/a-new-security-header-referrer-policy/