obviously registering the DLL is not required but of course that also has nothing to do nor does it help with your problem

the error you are getting is that it cannot load the DLL. In most cases this means that either the DLL is not accessible (path or rights) or any of its dependencies is not accessible

i would test with a non webapp test first to take the user rights issue out of the equation (or you can temporarily assign an admin user to your webapp service to see if it is a user rights issue)