Take a look at Josh's blog post
. Probably, this is your case...
See the Error section:
While developing your module, you may encounter the error “Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information”.
This problem can occur if you are modifying existing classes in your project as they may be cached in Sitefinity.
Rebuilding and restarting your website, as well as deleting your browser cache should resolve this issue
I hope this helps,