I've had this problem repeatedly with comcast website, and here's how I fixed it. I did try to do the " Safari>Preferen ces>Security>Sh ow Cookies", but could not locate it.

So I went to Safari>Preferen ces>Privacy>Cookies and other website data: REMOVE ALL WEBSITE DATA, then click on "DETAILS", and remove the Cookies on the specific offending website only. After doing this, I was once again able to...