No assembly found containing a Startup or [AssemblyName].Startup class

No assembly found containing a Startup or [AssemblyName].Startup class. To disable OWIN startup discovery, add the appSetting owin:AutomaticAppStartup with a value of "false" in your web.config.

Possible Solutions

When installing the Identity Sample NuGet package and then trying to do a find replace in the project to change all of the instances of IdentitySample back to the namespace of the project, depending on what is replaced and what is not, this may cause the above error. To solve this error open the Startup.cs file at the root of the web project and look at the namespace this class is in. Next open the web.config file and locate the tag. The [Namespace] portion may be IdentitySample or something else. if you are getting the above error chances are that it is different from the namespace that you saw the Startup.cs file was in. Change the namespace in the owin:appStartup tag to match the namespace of the Startup.cs class. This should fix the issue without turning off the AutomaticAppStartup as the error suggests which can cause other issues (primarily No owin.Environment item was found in the context errors).