'WebForm_PostBackOptions' is undefined after upgrate to 2.0?

[Dave]

Hi, I migrated ASP.NET 1.1 to 2.0. Now, on a form that uses
requiredfieldvalidator controls, I get:

Microsoft JScript runtime error: 'WebForm_PostBackOptions' is undefined

If I remove the validation controls, the form works. I've read some posts
that the WebResources.axd which handles the 2.0 javascript isn't being
downloaded properly etc. but I have yet to find a solution. Any suggestions
or official MS article would be great.

Thanks, Dave.