Which external authentication provider are you using? I've had some issues with the Azure AD login with openId Connect. Make sure all the settings in your Startup.Auth are correct. Below you can see my configuration for the 'OpenIdConnectAuthenticationOptions'. Also make sure that the 'MetadataAddress' uses the correct URL.
private OpenIdConnectAuthenticationOptions CreateOptionsFromPolicy(string policy)
{
return new OpenIdConnectAuthenticationOptions
{
// Generate the metadata address using the tenant and policy information
MetadataAddress = string.Format(CultureInfo.InvariantCulture, GlobalsHelper.AadInstance, GlobalsHelper.Tenant, policy),
AuthenticationType = policy,
// Set any properties required by your authentication service
ClientId = GlobalsHelper.ClientId,
ClientSecret = GlobalsHelper.ClientSecret,
RedirectUri = GlobalsHelper.RedirectUri,
PostLogoutRedirectUri = GlobalsHelper.PostLogoutRedirectUri,
AuthenticationMode = AuthenticationMode.Passive,
ResponseType = OpenIdConnectResponseType.IdToken,
Notifications = new OpenIdConnectAuthenticationNotifications
{
AuthenticationFailed = OnAuthenticationFailed,
RedirectToIdentityProvider = OnRedirectToIdentityProvider,
},
};
}