I have the same problem, no login possible with Safari 5, OS X 10.6.4, cookie settings changed to allow all did not help. I have to browse the forum with IE in the virtual machine, how absurd.
Right now at home it was working for me, too. At work it was not possible. But at home I use Safari 4, at work Safari 5. I will install Safari 5 now and check it at home, too.