Can you take a screenshot of whatever message accompanies the site blocking and post it here?
messages like those given below
Hi Sorry, I have uninstalled Mcaffee Family Protection from MAC, now able to use sites ok. Needed to access to order stuff
Okay. I can't see the error messages anyway because I'm not logged in with Family Protection. And this was a Family Protection issue, not related to SiteAdvisor.
If this ever happens again I would say it's down to the FP restrictions being too tightly enforced or wrongly set up.