I found the problem. My version IPB 3.1.2, FURL on.
The problem in FURL's.
There is a call FURL check function in ipsRegistry::instance()->init().
Before exit init() function it check FURL:
/* Have we entered an incorrect FURL that has no match? */
if ( ipsRegistry::$settings['use_friendly_urls'] AND self::$_noFurlMatch === true )
{
self::getClass('output')->showError( 'incorrect_furl', 404, null, null, 404 );
}
else if( isset(ipsRegistry::$request['act']) AND ipsRegistry::$request['act'] == 'rssout' )
{
self::getClass('output')->showError( 'incorrect_furl', 404, null, null, 404 );
}
That's why it pull 404 error and include a lot of forum html.