As long as your cookies are set as noted in the cookie FAQ and your
vb license number is correct your cookies should work fine.
Example setting both to
.yoursite.com
means the cookie is readable across every subdomain on the site so it would be impossible to be logged out which is why our facts state how to set the cookie like that.
If your creating cookies with domains like below and your urls flip flop between the www you will of course be logged out because the cookie domains are different which is the number one mistake people make with cookies.
yoursite.com
and
www.yoursite.com