Fatal Error: Unsupported Version of Windows Presentation Foundation

In case anyone else runs into this problem here is the solution.

The customer has XP Pro SP3 32bit as well as .NET 3.5 and AutoCAD MEP 2009 SP2.
I made a DLL in VB.NET using VS 2008 and I was not able to reproduce this crash in my test environments.

Eventually the customer found that he had placed the DLL on a network location and that was causing this error.