does ffmpeg have rtsps support? maybe check with ffprobe or ffplay?
thinking about it though, rtsp (and i presume the rtsps variant too) sets up 2+ connections for media: the first is a rtcp control stream, and it also provides a SDP stream definition file (which your client then uses to set up media sessions for the resources defined in there). it might also be worth dumping the SDP and seeing if you can see if the session setup for that works directly