One of my WinForms projects has recently stopped letting me add data
sets to forms or components through the visual designer.  When trying to
drag a dataset onto the designer I get "Unspecified Error", but no
additional information.

I can add the data sets through code, and they even show up through
Intellisense.  Adding them to InitializeComponent() does not bring them
up on the designer.  At runtime, however, they function as needed.

This only happens on one specific project in my solution, on all
existing forms and components as well as any new forms or components.
Everything works fine on the numerous other projects in the same
solution, using the same shared assembly full of typed datasets.

Any ideas on where to look for the problem, or even how to solve it?

--Mike