Same issue on my 55KU6409 after forced Plex app update on Februrary 15th.
What helped me so far: uninstall Plex app, install Plex beta app (Plex TV v2 Beta for Samsung 2016+ UHD Devices) and update Plex server to version 1.14.1.5488. At least now the app starts again and is able to play videos. Restarted the app and the TV, started playback where I left of several times - so far it worked.
