Do you have any custom dlls\widgets that reference Sitefinity dlls. It may be that that is throwing the error.
To ensure you have the right dlls delete all the Sitefinity ones in the bin directory and then from the Project Manager exe location you will find a folder called _empty project.
From the bin directory there take all the SF dlls and copy to your bin folder.
Do you have the site in a VS soln? Its much easier if you do.