I recently faced this issue while I was trying to launch my ASP.NET MVC application from Visual Studio 2017 but I was unable to do that because Visual Studio was always showing me a popup with below error message.

Unable to start debugging on the web server. The remote server returned an error (404) not found.

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Issue Root Cause

After some struggling, I found the root cause of the issue & it was some configuration settings in web.config file that was preventing Visual Studio from launching the application. And below was that setting - a requestFiltering tag.


<security>
      <requestFiltering>
        <requestLimits maxAllowedContentLength="2147483648" />
        <verbs allowUnlisted="false">
          <!--Allowed Verbs-->
          <add verb="GET" allowed="true" />
          <add verb="POST" allowed="true" />
        </verbs>
      </requestFiltering>
 </security>

Solution

Just add below tag inside verbs node. This will fix the issue & will allow Visual Studio to launch your application.

<add verb="DEBUG" allowed="true"/>