IFF you're using an updated Internet Explorer, changes in how the security zones are defined may cause your browser to accept only a temporary cookie from mudcat when you log in, and the cookie won't be there when you leave and come back.
To avoid logging in each time, you need (in IE) to go to Tools|Internet Options. On the "Privacy" tab click the "Sites" button, type "mudcat.org" (without the quotes) in the box and click "Allow." That should put the 'cat in your list to allow permanent cookies, so you'll stay logged in when you leave and return.