I use a config where my mantis server (local ip: 192.168.1.4, no host name) is behind another server (public host name: myserver), redirected through an Apache proxy redirect (RewriteRule with [P]).
On the mantis server, the HTTP_HOST is then 192.168.1.4, which is exact ; but the visible hostname should be 'myserver', for use in e-mails, etc., which is not the case currently (it displays 192.168.1.4).
The 'real' hostname is only accessible through HTTP_X_FORWARDED_HOST. So I suggest add a test in config_default_inc.php, line 71, to first test the existence of $_SERVER['HTTP_X_FORWARDED_HOST'] and use its value for $t_host if available (it works perfectly for me).
I precise I am not an expert in Apache configuration, so maybe my server config is incorrect - but I am not sure...