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It seems an issue related with Google+ shut down..
To ASP.NET Core 2.x try the workaround explained in https://github.com/aspnet/AspNetCore/issues/6486
services.AddAuthentication().AddGoogle(o => {
o.ClientId = _configuration["CLIENT_ID"];
o.ClientSecret = _configuration["CLIENT_SECRET"];
o.UserInformationEndpoint = "https://www.googleapis.com/oauth2/v2/userinfo";
o.ClaimActions.Clear();
o.ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "id");
o.ClaimActions.MapJsonKey(ClaimTypes.Name, "name");
o.ClaimActions.MapJsonKey(ClaimTypes.GivenName, "given_name");
o.ClaimActions.MapJsonKey(ClaimTypes.Surname, "family_name");
o.ClaimActions.MapJsonKey("urn:google:profile", "link");
o.ClaimActions.MapJsonKey(ClaimTypes.Email, "email");
});
Source:stackexchange.com