again no luck unfortunately. The strange thing is that both Apaches are configured the same so I would expect the same problem with both. This not being the case, I'm thinking the problem is not with Apache but with Horde or elsewhere. However Horde was copied from one server to another (it's actually the original install that's giving the problem so that rules out an error when copying across).
Do you have a language defined elsewhere? /etc/sysconfig/httpd, or /etc/httpd/conf.d/horde.conf, .htaccess, anything like that?