Thank @murattkn3
But this evening when using cable the Plex app on my LG 55UK6100PLB refused to connect works fine for Samsung smart tv, Apple, Android and Windows users.
What is really going on with this bad software???
I could read that downgrading the software to the previous version ain't easy.
I have spend like an hour resting the plex app, reinstalling it and so on still the same playback error and when I play the same contain that has worked before on this tv it runs fine so there is something fishy with this software.