Various solutions to this problem can be found here:
http://forums.aspfree.com/net-development-11/parser-error-message-could-not-load-type-17894.html
For me, deleting dlls usually works (you might also need to go into the temporary asp.net files directory inside the Microsoft.Net framework directory and delete all files there as well...this may require restarting or even rebooting the server).