I've been trying to debug a pesky "invalid callback or postback argument" error for several days now and VS has suddenly started throwing this "unsupported pseudo" error. ASP:hiddenfield is listed in the tool box. If I comment out the hidden fields in question from the HTML file and compile, the "unsupported pseudo" error disappears. When I uncomment them, it recurs. I've tried deleting the controls and recreating them but still get this error.

Can anybody suggest what I need to do to correct EITHER error?