Passing observable through 2 methods does not work

Question

Current behavior

I'm getting the following exception when I pass up an observable over 2 methods:

TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at HttpRequest.serializeBody (http.js:916)
    at Observable._subscribe (http.js:2196)
    at Observable._trySubscribe (Observable.js:172)
    at Observable.subscribe (Observable.js:160)
    at Object.subscribeToResult (subscribeToResult.js:23)
    at MergeMapSubscriber._innerSub (mergeMap.js:132)
    at MergeMapSubscriber._tryNext (mergeMap.js:129)
    at MergeMapSubscriber._next (mergeMap.js:112)
    at MergeMapSubscriber.Subscriber.next (Subscriber.js:90)

If I call the method post2 (see below) directly in the login function then everything works and I see the success message.

Expected behavior

No exception should occur.

Minimal reproduction of the problem with instructions

I have the following template & method: <button (click)="login()">Click me</button>

inside the login method:

public login(): void {
  this.webService
    .post('auth/login', { username: this.username, password: this.password })
    .subscribe((data: any) => {
       console.log('sucess');
     }, (err: any) => {
        console.log('errror');
  });
}

And then in the web service:

public post(url: string, body: any): Observable<any> {
  return this.post2(url, body);
}

and the 2nd method:

public post2(url: string, body: any): Observable<any> {
  return this.http.post('/auth/login', body);
}

If I call the method post2 directly in the login function then everything works and I see the success message.

What is the motivation / use case for changing the behavior?

Environment

Angular version: 5.2.7

Browser: - [X] Chrome (desktop) version 66