I had the same problem and what worked for me was this thread -
post number 23.
(Thanks Website Rob! )

Basically what I did was follow what that thread said which is:

1) Downloaded the html_output_admin.zip and extracted html_output.php file
2) Opened the file and added the additional lines (highlighted in green) in the correct place:

// Output a form
function zen_draw_form($name, $action, $parameters = '', $method = 'post', $params = '', $usessl = 'false') {
$form = '<form name="' . zen_output_string($name) . '" action="';
if (zen_not_null($parameters)) {
if ($usessl) {
$form .= zen_href_link($action, $parameters, 'NONSSL');
} else {
$form .= zen_href_link($action, $parameters, 'NONSSL');
}
} else {
if ($usessl) {
$form .= zen_href_link($action, '', 'NONSSL');
} else {
$form .= zen_href_link($action, '', 'NONSSL');
}
}
$form .= '" method="' . zen_output_string($method) . '"';
if (zen_not_null($params)) {
$form .= ' ' . $params;
}
$form .= '>';

/** BOF Security Patch v138 20090619 **/
$form .= '<input type="hidden" name="securityToken" value="' . $_SESSION['securityToken'] . '" />';
/** EOF Security Patch v138 20090619 **/

return $form;
}


3) Saved the file and overwrote the existing html_output.php file in admin/includes/functions

Worked for me so far so hope it works for you!