then try login out even before login in by adding
?action=logoutto the address.
if that doesn't work, clear your cache and your cookies
This is a common problem, and there are no real solutions for it yet. The only solution working for almost everybody, is to switch from useing IE to Mozilla FireFox. It is a browser problem for sure, and there is probably some cookie-related setting which should be changed. It would be great if you would try and change some settings, and if you will find some solution, please post it at the forums