I have isolated the exact post content that causes the BBCode::Parse to hang. I can’t post the original content, but I made this version replacing the text with lorem ipsum, and it also hangs:
$field =<<<EOD
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla dictum ligula sit amet massa condimentum gravida. Quisque qutris imgdper diet dolor. Nulla quis ante id elit lacinia commodo. Maecenas convallis felis at libero fringilla, feiat.<br><br><br><i><b>[EDIT: Aenean aliquet velit est, ac sodales "tortyor aliquoam vitae" urna sit amet dui rutr "OpenGL Uiot" Tenentis in faucibus orci luctus et ultrices posuere cubilia Curae Donec fringilla velit non too much
EOD;
If you remove the words “too much” from the end then it parses ok.
This is the full code that I put into a theme page template to run this test in isolation:
echo "<h2>Begin the parser test</h2>";
require_once( WP_PLUGIN_DIR . '/bbpress/bbpress.php' );
$field =<<<EOD
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla dictum ligula sit amet massa condimentum gravida. Quisque qutris imgdper diet dolor. Nulla quis ante id elit lacinia commodo. Maecenas convallis felis at libero fringilla, feiat.<br><br><br><i><b>[EDIT: Aenean aliquet velit est, ac sodales "tortyor aliquoam vitae" urna sit amet dui rutr "OpenGL Uiot" Tenentis in faucibus orci luctus et ultrices posuere cubilia Curae Donec fringilla velit non too much
EOD;
require_once( bbpress()->includes_dir . 'admin/parser.php' );
$bbcode = BBCode::getInstance();
echo html_entity_decode( $bbcode->Parse( $field ) );