Q:

auth0 callback url does not exists

So i am basically following this auth0 tutorial: 
https://auth0.com/docs/quickstart/webapp/aspnet-core/01-login
https://github.com/auth0-samples/auth0-aspnetcore-mvc-samples/tree/master/Quickstart/01-Login
on my localhost and my test-environment the login completely works. 
Although the error i am getting now on my staging environment also rings a bell, and had that before. But I do not know how I fixed it at that point.
During logging in it gets redirect to the callback url => callback, and not following my redirect url as i am setting it.
as in the accountcontroller
[HttpGet]
[Route("login")]
public async Task Login(string returnUrl = "/home")
{
    await HttpContext.ChallengeAsync(Auth0Constants.Auth0Scheme, new AuthenticationProperties
    {
        RedirectUri = returnUrl
    });
}

so the big question; what am I missing?
EDIT:
looking at the network traffic:
the one that does work:

I see that I have 2 authorize calls (not sure why?).
and in the one that is not working:

where does the second Login call come from..?

A:

it was due to the encryption. When running multiple instances the encryption key is different:
when keeping track of the key it is fixed:
https://docs.microsoft.com/en-us/aspnet/core/security/data-protection/implementation/key-storage-providers?view=aspnetcore-2.2&tabs=visual-studio

