Is there anything logged in your LOGS directory?

Also, I doubt this would make a difference, but make sure that your "sideboxes/bootstrap/search_header.php" has a unique Sort Order variable, just in case there's some weird conflict there.

I did a test and turned off my "sideboxes/bootstrap/search.php" but I am still able to see my "sideboxes/bootstrap/search_header.php" load just fine.