When running behind an apache reverse proxy, the code will not work because the reverse proxy will "correct" the code by adding a </form> just behind the <form> on line 308 and delete the </form> on line 367. The validate button will thus be outside the form and will have no effect.

This can be corrected easily by moving the form opening tag to line 264 :
$usersummary = "<form method='post' action='$scriptname'><table width='100%' border='0'>\n<tr><td colspan='8' class='header' align='center'>\n"

and moving the closing form tag after the closing table tag (i.e. switching lines 367 and 368.