XF 2.1 Viewing unknown page

I meant hover the "viewing unknown page" text, though the error element would be relevant -- as mentioned, this generally does indicate they're viewing a page that they can't view (or is not found, etc).
 
We have just upgraded from XF1.x to XF2.1.3 and are seeing this.

Robots seem to have correct data saying what they are viewing but the majority of Guests and Members have the "Viewing unknown page" message.

Tooltip says: XF\Pub\Controller\Error::DispatchError

No errors are logged in the XF Server error log.
 
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