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Clear your cookies or try the website in incognito/private mode. You probably have an old cookie for the same domain, but with invalid content.
If you are running in IISExpress and the soultion has the same url/port I have had an issue like this. As Johan said clear Cache and delete cookies and restart the application.
I am testing an upgrade from 7.11 to 8.1.0. I made a copy of the database. I updated the nuget packages, the database, and the configuration. When I run the application I am getting a 401 error.
Any idea of what is going on?