Trouble logging in

I'm using http://www.domain.com as Base URL, but cannot login using it. When I try to login using www.domain.com I get "Login successfully. Redirecting...", but then, still it says "You are not logged in".

When I try using only domain.com to login (with the same Base URL, http://www.domain.com) it logs me in successfully and redirects me to Base URL ([b]www.[b]domain.com). Now, trying to logout says "Logged out. Redirecting...", but still it says that I'm logged in as my username.

Using only domain.com works as I want it to work, but I can't get it to work using www. in front of domain.com I've checked cache_login.php and it changes when I change Base URL in the administration.

Any clues?