That is not a common error, are you using an install of pdftk on your own server? Is it possible for you to provide a temporary login for us to take a look at the plugin? If so, please send to support(at)formidablepro2pdf.com
Error is being caused by the use of Adobe Signature Field in the PDF file. Change your pdf form signature fields to regular text fields as our plugin does not work with the Adobe Signature Field – we just use a regular pdf text field to map Formidable Pro Signatures to pdf forms.