Always nice to see our old friend HRESULT in an error message.
Ran into this today when converting some .NET3.5 projects to .NET4.0 and at the same time upgrading some 3rd party libraries.
Seems that downloaded files get a blocked attribute set on them that prevents the usage of them. In Visual Studio 2010, the HRESULT: 0x80131515 is shown when linking to dll’s that are blocked.
To come around this, open Windows Explorer, navigate to the folder hosting the dll. Right click on it and select properties. There should be a Unblock button in the dialog window. Click on it and SGEN should stop complaining.
Hope this helps : )