This type of error popped up when porting a Visual Basic 2003 web project to become a Visual Studio 2005 web application project.
This was due to Webapplication1.dll being installed in wrong folder. Quick fix was to move Webapplication1.DLL from application folder into bin. Refresh the browser and it loads fine.
The longer term fix is in the setup project, right-click, View, File System - then move the DLL from the web app folder into the bin folder