Creating first topic on a user talk page (tested on office.wikimedia.org) fires an AJAX request for action=flow&submodule=new-topic with a ntreplyTo parameter which the API is rejecting.
{"warnings":{"main":{"*":"Unrecognized parameter: 'ntreplyTo'"}}, .. }
Another (on replies) is:
"Unrecognized parameter: 'reppostId'"