Hi,
I am having the exact same problem as the original poster, and the posted fix did not solve it. In addition to the login loop, sometimes it affects my ability to submit payment or even create an account. In all cases, I get stuck in a loop, where I submit my info (username/password, credit card info, etc), but instead of moving on to the next screen, I am brought back to the same page where I need to enter my info.
I have tried:
-clearing cache and cookies
-resetting Safari
-repairing permissions
-resetting the keychain as described above
-accessing the sites via Firefox, which always works
Safari is set to "Always accept cookies".
Since Firefox works, I do have a workaround, but it really bothers me that Safari (my preferred browser; I only downloaded Firefox after these problems started) cannot access these sites. And it seems that I discover a new "nonworking" site every day. My computer is about a year and a half old, and it is only in the last few weeks that these problems started.
Any help would be greatly appreciated!
Thanks,
Erin