After logging in users are immediately thrown back to the login screen
The login screen keeps reloading constantly

If looking at the Horde log file you see a successful login attempt, and the login screen doesn't contain any error message, or the login screen keeps reloading while you are using the "Automatic" authentication driver, e.g. during the initial setup.

This happens if sessions aren't working properly for some reason. These are possible troubleshooting steps and their solutions:

Check that PHP sessions are working at all using the sessions test on the /horde/test.php test script.
If not, make sure that your browser accepts cookies. Otherwise check http://www.php.net/manual/en/ref.session.php for more information and hints.
Try logging in with the default PHP session handler if you configured a custom session handler in Horde's setup.
If this works, something is broken with your custom session handler setup or backend. Continue troubleshooting there.
If using Horde 3.1.2 or later, change the configuration to allow non-cookie, url-based sessions.
If this works, cookies aren't stored in your browser. This could be caused by the browser configuration or a faulty session cookie setup, see the next point. If you already locked yourself out of Horde, edit the configuration file horde/config/conf.php manually and set $conf['session']['use_only_cookies'] = false;.
Make sure that you are using the correct cookie domain and path settings in Horde's setup.
Cookies are not stored in the browser if you don't use a FQDN (fully qualified domain name), e.g. http://localhost/horde/, or if the specified cookie path (/horde by default) doesn't match the webroot of your Horde installation, e.g. http://webmail.example.org/, http://www.example.org/horde-3.1.2/.