What kind of problem can cause a TypeLoadException?

Question

I have a big and bloated software and I want to add a new GUI element to it. The GUI element was written using XAML and WPF. I created the UI element in a separate assembly,

Answer 1

Just to add another answer to this:

we had a C++ client calling a .NET COM dll.

The developer of the .NET dll renamed the namespace enclosing the C# classes, and although the progid remained the same in both the class attribute and in the C++ calling code, the runtime failed to load the object resulting in this error.