Ok, so there must be something wrong with the horde code or rewrites rules, especially as this happens only on this one URL. If it would be a general rewrite issue e.g. caused by ispconfig, then no URLS in the /horde directory would work as ispconfig is not applying any rules specific to horde or one of its subdirectories.
It seems as if horde tries to rewrite to a file /var/www/horde/nag/t on the disk instead of https://cleandrum.com/horde/nag/t
which then gets rewritten to a php file. You should check the .htaccess file in /var/www/horde with the horde devs, maybe there is an option that needs to be activated or the URL is rewritten in the wrong way inside that file.