When you click the Unknown Location, where do you end up? What link?
Maybe you have "ErrorDocument 404 misc.php?google_seo_error=404" directive in your .htaccess without actually having Google SEO 404 enabled. So either enable Google SEO 404 or remove that directive from .htaccess.
Also the reason why everybody is showing that page is because you have images missing in your forum. So everyone who views your forum, follows it up with a request to the missing image, which can't be loaded => hence it ends up in 404 error land and that's also what shows up in location then since it goes via misc.php...
I am fixing it in his AdminCP