Microsoft recently introduced new ASP.NET Identity - replacement for old (Simple)Membership. Unfortunately, I can't use this new membership system in my old project because it throws System.InvalidOperationException: No owin.Environment item was found in the context. This is a known bug, but Microsoft keeps silence about this issue. The easiest way to reproduce this bug - it's to create a new web application (MVC, WebForms or WebApi - doesn't matter) in VS 2013 (with Web Tools 2013 Preview Refresh) and then go to the login page. It will work. Then change namespace in your application to anything else than original namespace and login page will throw that error. Changing namespace back to original (the one you used at the creation of a project) will solve this problem.
It looks like .net stores somewhere something related to the original namespace, but I can't find what and where, it's not in the project folder. I know that stackoverflow is not a place for a bug reports, I just hoping that someone already found a solution for this issue or maybe people involved in the development of ASP.NET Identity will see this.
Most likely it cannot find the OWIN Startup class. The default convention for the Startup class is [AssemblyName].Startup. If you're no longer following that convention you'll need to specify the full name of your Startup class in the Web.Config.
The next release of Microsoft.Owin.Host.SystemWeb package now throws detailed exception messages when the Startup class cannot be found.