To disable OWIN startup discovery, add the appSetting owin:AutomaticAppStartup with a value of "false" in your web.config.

I made a dummy ASP.NET web forms app in Visual Studio 2013 and stripped everything out it but one web form and then I got this error. As suggested, the solution is to put the following the Web.config's appSettings:

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