I am getting the same exception. It was waorking with JBoss 4.0.5 (Seam 1.2.1), it fails on JBoss 4.2.2 GA (again Seam 1.2.1, JSF RI 1.2).
Anyone has a solution for this?
Ok, seems I fixed it by removing javax.faces.STATE_SAVING_METHOD client setting from web.xml. Looks like it behaves differently from JSF 1.1.