Http failure response for localhost api request: 0 Unknown Error - Angular

I'm trying to create a communication to my Java Jetty Backend from my Angular application. When I try to execute my request I receive the following error:

My code on client side: (Angular 7.2.1). I'm also using a HttpInterceptor for authentication that should work. I'm also running the code in development mode with ng serve.

@Injectable({
    providedIn: 'root'
})
export class NgHydrantService {

    constructor(private http: HttpClient) {
    }
  
    public register(entity: IEntityDescription): Observable<StandardResponsePacket> {
        let packet = new RegisterEntityRequestPacket(entity);
        return this.http.post(this._apiUrl, packet.toJson())
            .pipe(
                map(value => {
                    console.log('register result:', value); //<-- never executed
                    return <StandardResponsePacket>HydrantPackage.fromJson(value)
                })
            );

    }
}

//The interceptor
intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        // add authorization header with basic auth credentials if available
        if (this.user != null) {

            const clonedRequest = request.clone({
                headers: request.headers.set('Authorization', `Basic ${this.user.getAuth()}`)
                                        .set('Accept','application/json')
            });
            //This debug line works and looks good!
            console.log('NgHydrantAuthInterceptor#intercept', clonedRequest);
            return next.handle(clonedRequest);
        }

        return next.handle(request);
    }

My Code on server side: (Jetty-9.4.14.v20181114) that runs on localhost.

public final class PacketHandler extends AbstractHandler
{
  @Override
  public void handle( String target,
                    Request baseRequest,
                    HttpServletRequest request,
                    HttpServletResponse response ) throws IOException
  {
    try
    {
        // Declare response encoding and types
        response.setContentType( "application/json; charset=utf-8" );
        // Enable CORS
        response.addHeader("Access-Control-Allow-Origin", "*");
        response.addHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, PUT, DELETE, HEAD");
        response.addHeader("Access-Control-Allow-Headers", "X-PINGOTHER, Origin, X-Requested-With, Content-Type, Accept");
        response.addHeader("Access-Control-Max-Age", "1728000");
        //... more stuff
    }
    finally
    {
        // Inform jetty that this request was handled
        baseRequest.setHandled( true );
    }
  }
}

Things I checked:

• During research some people reference problems with CORS (that's why I added the header entries in the server side code)
• The same request in Postman works without any problems
• There are no logs on the server side

My question is about a possible solution to get responses from my server during development.