Could not load file or assembly 'System.Core, Version=18.104.22.168, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' or one of its dependencies. The system cannot find the file specified.
I've had about all I can take from this error. It occurs randomly, out of nowhere, when I try to compile anything in VSTS and a Silverlight project (of any type, xap, dll, etc) exists in that compile sequence.
Up until now the only fix I have found is to exit all instances of Visual Studio and load them back up. This makes the error go away for at least one compile.
Well, having put up with just about enough out of Silverlight, I have another fix that is technically not something you would normally want to do. Add System.Core.dll to the Global Assembly Cache (GAC).
Search for the System.Core.dll relating to Silverlight (likely at C:\Program Files\Microsoft Silverlight\2.x) and use the gacutil.exe to add it.
After doing this, the problem has not reoccurred for me.