Somewhere in the html form you simply missed the opening quote of input name attribute(s).
Example
<input type=”text” name=last_name“>
Just find and use the right syntax for happy coding.
You can find the code for tracking disallowed characters are in system/core/input.php file
function _clean_input_keys($string)
{
if (! preg_match("/^[a-z0-9:_/-]+$/i", $string))
{
exit('Disallowed Key Characters.+'. $string));
}
return $string;
}