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.
This is a bug that was introduced in a recent (2018) version of W3 Total Cache. The Default value for the Referrer policy under the Browser Cache Settings became “” (blank) in an update in early 2018. The actual setting should be “origin”.
Referrer Policy - Origin
You can find my recommended Browser Cache Settings here. In particular, see the screenshot shown in Step 6 of the Browswer Cache settings section.