CORB issue using .NET Core 2.2 and Angular 7

After spending a lot of time getting the CORS issues sorted, we're now stuck with CORB errors. 

We use a Cors service like this:

services.AddCors(options =>

            {

                options.AddPolicy("CorsPolicy",

                    builder => builder.AllowAnyOrigin()

                    .AllowAnyMethod()

                    .AllowAnyHeader()

                    .AllowCredentials());

            });

And then using httpClient like this:

        const httpOptions = {

            headers: new HttpHeaders({

                'Content-Type': 'application/json',

                 'Authorization': 'Basic ' + btoa(this.userEmail + '/token:' + this.myToken)

            })

        };

        var theResult = this.http.get<any>(this.url + 'tickets.json', httpOptions).subscribe((res: any[]) => {

            res => console.log(res)

        })

Chrome, with a CORS extension installed and running, still gives this message:

Cross-Origin Read Blocking (CORB) blocked cross-origin response https://starware.zendesk.com/api/v2/tickets.json with MIME type application/json. See https://www.chromestatus.com/feature/5629709824032768 for more details.

We have checked:

• IIS Express configuration, making sure CORS headers are delivered.
• Ng serve disabling origin with argument to server
• Ng serve proxy configuration
• Chrome CORS extension
• Service AddCors() configuration and app.UseCors() allowing any header and origin
• Component DisableCors parameter
• X-Content-Type-Option=nosniff
• Jsonp call (you cannot authenticate to Zendesk anymore)

I do not know how to solve this. Any help is appreciated.