Error Reporting in Silverlight is really bad. One of the most common error you see whiled developing a Silverlight application
This error mostly occurs due to following,
Version Change in assemly while you are working in the same code (i.e. you used version x initially and later on you changed it to version x + y ). Some Assemblies in 3rd party control when upgraded do not support few older properties or methods.
Your XAML code is referring to some event which is not present in the code behind file. Make sure that all XAML code has relevant code in code behind file. (Remove any unnecessary Events from XAML and CS files)
Your XAML code has repeated the same code again, causing a clash.
Hope this helps