Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies. The module was expected to contain an assembly manifest

Question

I am facing with strange error when I deployed my application on IIS7 When I open my site in VS 2012 without changing its framework which is .Net 4.

Source Error:

Line 1:  <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="FrmUserLogin.aspx.cs" Inherits="VisionWebPortal.FrmUserLogin" %>
Line 2:  <%@ Register Assembly="AjaxControlToolkit"  Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>
Line 3:  <%@ Register Src="MessageBoxUsc/uscMsgBox.ascx" TagName="uscMsgBox" TagPrefix="uc1" %>

The problem is with Line 2 of Ajax assembly reference. When I deployed the same on my laptop, It is working fine with out error and It also works with VS 2010. Secondly there are two more websites on the same IIS and that have also the same Ajax tags which are working fine. I attempted the following but still out of luck:

1. Clean Solution and Rebuild
2. Deleted AjaxControlToolkit.dll reference and Re-add the reference
3. Give Full Permissions "Temporary Asp.Net Files" for IIS/User by modifying folder security permissions
4. Restart Visual Studio 2010 and IIS server
5. Checking App Pool version which is V4.
6. Uploaded whole project including Bin files

Answer 1

The problem was with Visual Studio versions. When you changed project file details by opening in different versions of Visual Studio like VS 2010 or VS 2012, then this types of errors appeared.