This really is one of Microsoft's finest error messages isn't it. :-)
I did a quick google for "Retrieve the LoaderExceptions property for more information", I suggest you do the same a quite a few people have the same error message, but with differing causes.
These range from different compilation target frameworks in release/debug to a referenced assembly being missing to just a plain old rebuild needed.
This[
^] seemed to have quite a few answers to the various causes.
I'll 5 your question, hopefully someone else will have a better answer.