I’m assuming you followed this quickstart which as part of the installation of the
Auth0-ASPNET package creates a
LoginCallback.ashx handler with associated code behind in your project.
The package creates the class by default under the root namespace of your project and also points the handler to that class (also within the same root namespace). Given that at install time both handler
*.ashx and code-behind
*.cs assume the same namespace I could not reproduce this error.
However, if for example you change the namespace on the code-behind file but fail to update the handler definition file then you’ll indeed get an error similar to the one you shown. You should ensure that both the code-behind and handler definition file refer to the same namespace.