You need to convert a PCL library to a full version of the framework. You may run into this scenario if you are running code on a server that you cannot patch and is not fully patched with the latest .Net patches. There is a bug where PCL libraries will not run on .Net Framework 4.0 sites that aren’t current in their patching (You will receive an error that goes something like this “Could not load file or assembly, 'System, Version=126.96.36.199...'”).
In an ideal world you would be able to have your hosting service apply KB2468871 (http://www.microsoft.com/en-us/download/details.aspx?id=3556). We don’t always live in an ideal world.
How to change your project from a PCL to a full .Net project: