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Recent Mantis versions (I'm using 1.2.4) send the HTTP header "X-Frame-Options: DENY" which causes the iframe embedding in Drupal to not work (Mantis does not show). Editing the core/http_api.php and either commenting out this line:
header( 'X-Frame-Options: DENY' );
or changing the DENY to SAMEORIGIN (if mantis is on the same domain as the drupal site), will fix it.