How to use Update Xml action to update another item

Hi,

I have two form libraries... named, for example, "Form A" and "Form B" in the same. I also have a workflow in "Form I".

When I create a form in Form A, it starts the workflow and it creates a new form in Form B.

After this new form creation (in Form B), I need to update its fields, using some information from the form created in Form A. I used the "Update Xml" action, but it didin't work out, returning the message: "... (401) Not authorized".

 How can I do this?

Thanks in advance.

Andre