How to catch an error on a Request, then open a modal, then retry when modal closes with RxJS

I think that you need to return something an observable even in the case of the 401 error:

public errorHandler(res: Response, ob: Observable<any>): Observable<Response> {
    if (res.status === 401) {
        let closedSubject = new Subject();
        this.modalService.open(new ModalConfig({
            content: LoginModalComponent,
            close: () => {
              closedSubject.next();
        }));
        return ob.retryWhen(() => closedSubject);
    }
    else {
        return Observable.throw(res.json());
    }
}

See this article for more details: https://jaxenter.com/reactive-programming-http-and-angular-2-124560.html.

Edit

The problem is that the second parameter of the catch callback isn't the source observable. This source observable corresponds to the value of its source property:

return ob.source.retryWhen((errors) => closedSubject);

See the working plunkr: https://plnkr.co/edit/eb2UdF9PSMhf4Dau2hqe?p=preview.

I guess retryWhen operator should help.