This FAQ applies to Version 6 only.
The calendar uses PHP sessions to store your login credentials and PHP needs a place to store the sessions information.
If you seem to be able to log in, but are immediately redirected back to the login screen, PHP sessions isn't properly configured on the server.
To resolve this, contact your web host provider and see if they can properly configure PHP sessions. They may give you a sessions path which you would then add to the sessionsPath.php file in the setup directory.