I was getting that message, and I isolated the problem down to the first lne in my .ascx file:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="ContactInfo.ascx.cs" Inherits="ContactInfo" %>

the Inherits="" class name did not exactly match the name of the code-behind class.

Check to make sure all your class names match in the master page, usercontrols and other pages.  That might help.