Sometimes your cookies files becomes corrupted or too full.

There are times that you will encounter login problems or are unable to view pages or use functions because your cookie file needs some cleaning.

Always clear your browser's Cache file and Cookies to give you a fresh start. Some people like to not clear these files because it helps keep their browsing history intact.

Even if you clear your cookies an often overlooked problem occurs when you have set your browser to ask for permission with cookies. Suppose you allowed cookies from certain sites and blocked some from other sites, it can happen that over time the site changes some codes, etc. Suddenly you can't get in or something does not work right. It might prevent you from using Chat or uploading photos.

The solution is to reset all your cookies permissions by deleting the allowed and blocked settings so that the browser has to ask for every site you visit again. This rebuilds the permissions file with good information.

Consider also changing to the FireFox browser which will work with both the Mac and PC. For more information on that go to the question on Browsers