Although Expression Blend and Visual Studio can work on the same solution / project files, they remain two separate IDEs and it is not uncommon for certain functionality to work correctly in the Visual Studio design-time environment and throw an error in the Expression Blend one. The Blend error output does not always help much with identifying the reason for the erroneous behavior:
This clearly does not give us enough information to track the problem down. Fortunately there is an easier way to detect the problem instead of wandering around the code in a manual “debugging” session – the Visual Studio debugger.
Here is a step-by-step guide that explains how to identify Expression Blend exceptions through the Visual Studio 2008 IDE:
Hope this helps.