I’m busy with Silverlight + SharePoint 2010 in Visual studio 2010.
All went well , but suddenly I’ve received an error with the mention of “ValidateXaml” task failed unexpectedly.
Ok reading the error furthur there was a mention of System.Windows not being found on the server.
That’s strange because the reference was right there.
The solution isn’t best case but it did the job for me.
Just go to c:program files(x86)references assembliesMicrosoftFrameworkSilverlightv3.0
Add System.dll, System.windows.dll and System.core.dll to the gac via gacutil.exe (gacutil /i “name of file of dll”).
My thanks to Gill Cleeren for finding this solution for me.