Change /home/httpd/html/horde/config/conf.php
//125Authentication
$conf['auth']['checkip'] = true;
from true to false
This is the write up about this setting in horde - Should we always store and validate the IP address of the client (as seen by the web server) in the session? Doing so will help increase security by making it harder for an attacker from another host to hijack the session
Test it to make sure, then create a custom-template to make the change permanent.
John