When I try to log in with Safari: I enter my user name, I enter my password, I click "Log-In" and then the fields for user name and password are blank again and I am not logged in.
Oh, and the problem does not occur with every Mac. I was logged in with the Mac at work today, so I thought that it was just a temporary problem, but now at home I am still not able to log in with Safari v. 5.0.1 (I have updated Safari since my previous post, but that didn't help).