Plex media player - connection reset by peer

Playback state and tracking all works fine, but trying to send controls from the cards results in

Failed to call service media_player/media_play_pause. ('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))

The Plex Server reports

Request: plaintext connection from 10.14.16.21:36896 rejected because secure connections are required

Secure connections are required on the server. Any ideas why playback state would show up, but sending commands fails? I would expect both to either work or fail, given the error reported…

Anyone? Not sure where I’m going wrong!