I slightly remember I have seen this issue before. I think it was a bug which should be fixed in the meantime, unfortunately I wasn't able to find the associated entry in out bug tracking system, so I cannot verify and/or provide details. In case you are running an older build of MWG (the tags state 7.1.6) my recommendation would be to perform an update to the latest version and see if the issue persists.
solved by updating to main release 18.104.22.168 (16052). Log lines containing status code 403 (blocked by MWG) now look like they should:
[...] 22.214.171.124 403 "GET https://accounts.google.com/o/oauth2/postmessageRelay?parent=http%3A%2F%2Fwww.tt .com HTTP/1.1" "Internet Services" [...]