Usually you can simply close your browser try again and it works.
However, it's possible that your server recently changed their Apache and/or PHP configuration.
I suggest that you upgrade to v1.77b, now available from the members area. Included with this release is a new login method implemented because of a similar problem on certain servers.
Simply replace your main ListMail files, except config.php, with the ones from the new ZIP and re-login to the program.