Hi Roberto
I've seen this before. I never completely resolved the cause. I think it may happen when the XML libraries you have on the client and those that you have on the server are different. Another thing to look at is whether you've recently upgraded your QPACs to later versions.
I would start by creating a completely new workflow, rather than try to fix the one you're currently using.
I'd also make sure that you have the right version of all libraries on both client and server, that you're using matching versions of server and client, and that you don't have two people trying to update the same workflow at the same time.
Good luck...
Howard
http://www.avoka.com