CauseIt means that you are accessing Horde through the ISP that has dynamic proxies.
ResolutionThe only solution is to turn 'checkip' feature off in Horde server-wide. Login to the Parallels Plesk Panel server via SSH and edit the /usr/share/psa-horde/config/conf.php file in your favorite text editor. Change the line:
$conf['auth']['checkip'] = true;
$conf['auth']['checkip'] = false;
Note, this modification cannot be configured per user basis and will affect all Webmail users.