Webapp works in vs 2010 but not vs 2015
<p>I have a WebApp that works ok when i run it from 2010, but fails with this message when i use 2015 :</p>

<pre><code>Access is denied. 
Description: An error occurred while accessing the resources required to serve this request. The server may not be configured for access to the requested URL. 
Error message 401.2.: Unauthorized: Logon failed due to server configuration.  Verify that you have permission to view this directory or page based on the credentials you supplied and the authentication methods enabled on the Web server.  Contact the Web server's administrator for additional assistance.
Version Information: Microsoft .NET Framework Version:2.0.50727.8689; ASP.NET Version:2.0.50727.8670
</code></pre>

<p>The project loads into 2015 without needing to be upgraded. Why doesn't this run in 2015?</p>

<p>Edit : to answer @Aristos, it uses webforms, and the URL is <a href="http://my_app:port_here/" rel="nofollow">http://my_app:port_here/</a> (set too a dynamic port), and the project/solution is a web application in VS. It runs from 2010 but not from 2015</p>