Notes/Domino 4 and 5 Forum

i am facing a problem, i believe, with Authors field.
There's a form with a authors field. a person can open a document in edit mode (where he is having author access and also a role which is listed in the authors field of the document). But after editing the document, when he saves it, a notes error, "You cannot update or delete the document(s) since you are not listed as an allowable Author for this document" is displayed.
Please suggest.