In Horde on a plesk server, if you get this error message it means your ISP has you setup so that your IP address constantly changes. To stop this error from happening, you need to disable the IP check in horde.
To do this, SSH to the server, then edit the following file in your favorite text editor.
Change this line
$conf['auth']['checkip'] = true; to $conf['auth']['checkip'] = true;
You should no longer get these errors when connecting from a computer whose IP addresses changes.