Error message: Your Internet Address has changed since the beginning of your Mail session. To protect your security, you must login again.

It means that you are accessing Horde through an ISP that has dynamic proxies. The only solution is to turn 'checkip' feature off in Horde.

To do this you need to edit the "%plesk_vhosts%\webmail\horde\config\conf.php" file. There should be two lines:

$conf['auth']['checkip'] = true;

Both lines must be changed to

$conf['auth']['checkip'] = false;

in order to prevent IP address check upon login and the problem described above.