For most of your files, unless you've customized something, you "want" them to say "files are identical".
In WinMerge, if you click "View", and uncheck "Show Identical Files", then you'll only see things that are "different", making the analysis process much easier.
I would have expected a problem in the header_php.php file, since the problem is only happening after you click Submit.
You should compare the /includes/functions files.
I would also compare your tpl_contact_us_default.php template file to be sure it's intact.



