Add the entry doubleclick.net to the URL filter, set to all machines. Give it a day or so of use and see if web browsing stops. You might be able to trigger this bug by uploading to a website (google for 'submit your image' or some such) or the upload section of a speedtest with the URL filter on.

Https sites, pings etc continue to work but nothing via port 80

May be limited to ufb