System.EntryPointNotFoundException when loading Sitecore 9.0

Question

On Sitecore 9.0, when I attempt to load any URL, I get an Owin startup exception with the following details:

System.EntryPointNotFoundException: The following errors occurred while attempting to load the app.
- No assembly found containing an OwinStartupAttribute.
- The discovered startup type 'Sitecore.Owin.Startup, Sitecore.Owin, Version=1.1.0.0, Culture=neutral, PublicKeyToken=null' conflicts with the type 'SolrNet.Startup, SolrNet, Version=0.4.0.2002, Culture=neutral, PublicKeyToken=null'. Remove or rename one of the types, or reference the desired type directly. To disable OWIN startup discovery, add the appSetting owin:AutomaticAppStartup with a value of "false" in your web.config. To specify the OWIN startup Assembly, Class, or Method, add the appSetting owin:AppStartup with the fully qualified startup class or configuration method name in your web.config.

What could be causing this?

Answer 1

This is an indication on Sitecore 9 that your Web.config has been overwritten. Check the deployed Web.config, replace it with the stock 9.0 version if needed, and/or ensure your Visual Studio solution is configured to deploy the proper Web.config. It could be that one was added to a web app project after installing a nuget package. If you're using TDS, try setting the Build Action to None on Web.config files which you do not wish to deploy.

Answer 2

I have resolved the issue with @techphoria441 answer.

Ensure that all your projects web.config Build Action is set to None.