401 The web site is blocked by administrator

I'm the administrator for a network and recently, I have been getting
reports of a 401 The web site is blocked by administrator message. I
have only blocked one site and the sites this message keeps appearing on
is not the site I've blocked via the D-Link DI-604 router. Two of the
sites that get blocked are staples.ca and cbc.ca (according to the
router, these sites are not blocked and I did not block them). Any help
as to why this is happening and any way to fix it would be greatly
appreciated. Thanks.

Chris