angular .NET CORE web api Http failure response 0 Unknown Error

I have been reading many posts related to the error "Http failure response 0 Unknown Error" here, but none of them seems the same as what I have encountered.

I have an angular 6 UI, and a .NET CORE (2.1) Web API as backend. All works perfect in development mode (UI and backend talk in VS Code and VS 2019 environment). Then I deployed backend API to the website server (public to internet), and change api url in the dev UI to point to the server web API url for testing. When sending a http.get, I got the error "Http failure response 0 Unknown Error"

error message

Many posts mentioned about incorrect configuration of CORS on the server side, but I don’t see it being mentioned in my error message and I do have configured Cors on server side.

Here are some more info:

1 the controller is annotated to [AllowAnonymous] to simplify the test

2 backend API site is self-certified (no CA). when browsing from Chrome, there is a warning of Not secure. I don’t think that it affects the consuming of web API.
chrome browsing

3 using postman to test against the server web API, I can call the methods without any error.

postman test

4 CORS configuration in web API startup.cs
(this is all code related to Cors config. did I miss anything?)

cors configuration

So what is possibly the cause of this error when I can run all smoothly in development environment, and Postman runs successfully against real server, but when I try to consume the real server web API from my UI dev mode, I get the error of "Http failure response 0 Unknown Error". Is it possible to explore more detailed error message anywhere?

I appreciate your help and suggestions and happy new year to all fellows.

Source: Angular Questions