I too am getting this error, on asp.net 3.5 with a registered control.
http://www.*******.com/controls/FormShield.aspx?__formShieldID=FormShield1&generate=image&properties=3AD01C0254B4BCE975B6A7F52B663F05809EA3F76FD275F6DFC9D36BE1B2745EA9338FD27FAA911DAE404C06BD37ED123DC6D0FE6ED78E2CF5E4DE6D4336F80DE58630E4B61C73F723516D1A100518BD