I was getting this error when trying to open a asp.net 2.0 page.
However other applications developed in asp.net 1.1 were running fine.
Then realized that both of them were using the same Application Pool, so created a new application pool and assigned this new application pool to my asp.net 2.0 application.
That solved the issue.
“An application pool is a process that responds to web requests under IIS. An application pool does not have a setting for what type of ASP.Net applications will be run in it. Instead, it loads the appropriate libraries when an ASP.Net application is loaded in the process. Because the libraries for ASP.Net 1.1 and ASP.Net 2.0 are similar, but not the same, the application pool cannot respond to requests for both types of applications at the same time. This can cause sporadic behavior if you are using the server at the same time as another developer and you have applications using different versions of the framework in the same application pool”