All app pool versions are v 4.0 but "It is not possible to run two different versions of ASP.NET in the same IIS process." error raise.

Hello,

We've got a very strange problem on our Windows Server 2008 R2 Standard (64bit) web server (iis 7.5).

All app pool versions are v 4.0 (and integrated Mode).

But When I reference 'Default Web Site', HTTP 500 error raise.

What I checked things, below.

1. when I changed 'DefaultAppPool' (Default Web Site's app pool) .net framework version to v 2.0, then error didn't occur.

2. when I change 'DefaultAppPool' .net framework version to v 4.0 and classice mode, then "It is not possible to run two different versions of ASP.NET in the same IIS process." error occured.

3. when I change 'DefaultAppPool' .net framework version to v 4.0 and integrated mode, then http 500 error occured.

<more informations>

1. SharePoint (v3) has been installed on this server.

2. so , When it was confirmed at the beginning, this server has two .net framework versions ( v2.0 , v4.0 ) app pool.

3. this server has three web site ( Default Web Site /  WebSite for sharepoint (at first v 2.0 (app pool's version) , it has already change app pool's version to v 4.0) / WebSite for Team Foundation Server (app pool's version is v4.0)) 

Please give me some advice..!