No OWIN authentication manager is associated with the request

After trying to enable owin & AspNet Identity to my Web Api project (in VS 2013 + .Net 4.5.1) I get the following error in each valid or unvalid(request to none exist controller) requests :

<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
No OWIN authentication manager is associated with the request.
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace>
at System.Web.Http.Owin.PassiveAuthenticationMessageHandler.SuppressDefaultAuthenticationChallenges(HttpRequestMessage request) at System.Web.Http.Owin.PassiveAuthenticationMessageHandler.<SendAsync>d__0.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() at System.Web.Http.HttpServer.<SendAsync>d__0.MoveNext()
</StackTrace>
</Error>

As I checked in debug mode, no exception is handled too! Also I realized that Configuration in Startup class is never called (indeed never caught by the debugger). here is the code for startup :

[assembly: OwinStartup(typeof(bloob.bloob.Startup))]

namespace bloob.bloob
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            ConfigureAuth(app);
        }
    }
}

I found the problem finally! After comparing line by line with a newly created project and finding no difference , I checked references on both projects and yes!... All the problem was from missing package :

Microsoft.Owin.Host.SystemWeb

I don't know why this packaged is missed in package installation phase but the strange point is that why didn't any build exception thrown? or no any dll reference error?

I originally created the project with authentication, but then decided to disable it. I had to remove this in the WebApiConfig.cs file. Make sure you have this if you intend to enable authentication.

        // Web API configuration and services
        // Configure Web API to use only bearer token authentication.
        config.SuppressDefaultHostAuthentication();
        config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

My case, it failed since this settings in web.config. Hope this helps someone to avoid it.

<appSettings>
    <add key="owin:AutomaticAppStartup" value="false" />
</appSettings>

I had the same problem. The package was not appearing in the NuGet Package manager. I added reference in packages.config:

 <package id="Microsoft.Owin.Host.SystemWeb" version="2.1.0" targetFramework="net45" />

And reference in the project file (xxx.csproj):

 <Reference Include="Microsoft.Owin.Host.SystemWeb">
  <HintPath>..\packages\Microsoft.Owin.Host.SystemWeb.2.1.0\lib\net45\Microsoft.Owin.Host.SystemWeb.dll</HintPath>
</Reference>