The module examines all the request variables for html tags, it could be that there are some other request variables that happen to contain them. It doesn't explain everything though.
I don't know the answer at present. Since I cannot replicate the problem it is quite hard to solve.
You could try commenting out lines 136 to 146 in the module helper.php file:
$regexp = '/[<\[]/'; //spam filter
foreach ($_REQUEST as $requestVar)// use raw request for this because JRequest filters out < tage
{
if(isset($requestVar)){
if (preg_match($regexp, is_array($requestVar)? $requestVar[0]: $requestVar )){
$lists->error = true;
$lists->error_message = JText::_("INVALIDHTMLTAGS");
return $lists;
}
}
}
Support 
