How to set the value cannot be null Parameter name: typeName when opening the XAML file?

We are using Silverlight 4 (we must use it .: p) with VS2010. AFAIK we have the appropriate tools and SDK.

When we try to open the XAML file, we immediately get a dialog box with an error message: "The value cannot be empty. Parameter name: typeName". Designer does not load. There is no way out in the output window, there are no build errors.

We are in complete loss of what to do.

This affects only some of our workstations. We are in a controlled environment, and reinstalling VS2010 is actually not an option. Again: p

Thank!