CAUSE
It means that you are accessing Horde through the ISP that has dynamic proxies.RESOLUTION
The only solution is to turn 'checkip' feature off in Horde server-wide. Login to the Plesk 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;
to
$conf['auth']['checkip'] = false;
Note, this modification cannot be configured per user basis and will affect all Webmail users.
Keywords: webmail Horde