[horde] Changing language on mail.domain.tld results in 404

[Jochem Meyers]

When changing the language in the dropdown of the login page, the refresh
leads to a 404 error.

We run horde on mail.domain.tld. When going to mail.domain.tld, the url
remains unchanged, and the login form is displayed.

Going to mail.domain.tld/login.php, a 404 error occurs.

When changing the language from mail.domain.tld/horde/imp/login.php, it
works perfectly.