IDX20803: Unable to obtain configuration from

Solved it by replacing, in appsettings.json :

"AzureAd": {
     "Instance": "<APP_Uri_from_Azure_Portal>",
...
}

To

"AzureAd": {
     "Instance": "https://login.microsoftonline.com/",
...
}

If anyone has this issue and is using the Azure B2C the instance is your root url for your azure b2c tenant. e.g. https://myroot.b2clogin.com/. The domain should be: myroot.onmicrosoft.com/

Make sure there is no https in the domain.

This worked for me.

{
    "AzureADB2C": {
        "CallbackPath": "/signin-oidc",
        "ClientId": "<app-registration-app-client-id>",
        "ClientSecret": "<secret-value>",
        "Domain": "<domain-name>.onmicrosoft.com/",
        "EditProfilePolicyId": "B2C_1_profile",
        "Instance": "https://<domain-name>.b2clogin.com/",
        "ResetPasswordPolicyId": "B2C_1_reset",
        "SignUpSignInPolicyId": "B2C_1_signupsignin"
    }
}