cant sign livecycle designer ES2 (9) form in reader x

Hi,
 
first of all, sorry for my english, I'll try to explain my issue as fine as I can.
My work tools are:
 
- my PC: windows 7 ultimate, adobe acrobat suite x
- client´s PC: windows xp sp3, adobe reader x
 
I have a VAT invoice form done that needs to be signed by client to authorize it. The problem is that when I save as reader extended enabling aditional features and try to open it in reader x i get the following error message:
 
"This document enabled extended features in Adobe Reader. The document has changed since it was created and the use of extended features is no longer available. Contact the author of the original version of the document."
 
I´m newbie to Livecycle: am I overrriding any fundamental step?
 
Any suggest?
 
Thanks in advance.