Office-js addin with webapi on Azure. Fetch error.

Windows 10  20H2, VS Community

Application is one office-js addin (typescript) + one Webapi core (c#) in a container, both components communicating with a fetch function thanks to cors. 

It works properly on my local computer.

I published both components on azure, the webapi component being derived as a container instance. I checked that the correct azure addin address was entered in the webapi startup module.

Here, each  individual component works  properly (proof with messages from the addin and with postman for the webapi)  but fetch function raises errors (message = "fetch error).

Could someone tell me what I did miss ?

Thanks in advance.