A bug fix was added to form_panel.module on March 11, 2013 - 13:10. This line in function theme_form_panel_table() was updated...

$kids = array_diff(element_children($form), array('form_id', 'form_build_id', 'form_token'));

It causes a form (under Drupal 6) that uses the theme e.g.
$form = array(
'#theme' => 'form_panel_table',
'#form_panel_table_attributes' => array('border' => 0));

not to be processed on submission (no validation or submission whatsoever is triggered). I do not know how this module works so I do not understand how array_diff is supposed to help... but when I change this line back to the way it was before the bug fix...
$kids = element_children($form);

then form validation/submission works.

Alan.