Our new LG smart TV (actually the 43UM7100PLB) connected over WiFi network to NAS device (network attached storage) of Western Digital (WD) MyCloud Ex2 which supports DLNA (digital living network alliance). Some movies were not playing on the TV correctly. The stated error message was Unable to play. This file can't be recognized. Which was weird because they actually were playing for several seconds and their library thumbnails were displaying ok. If I put the movie file on a USB they were playing well too.
It turned out that encoding of the video or its format was not a problem. I tried to connect the LG smart TV to the WiFi router and thus to the NAS over network cable instead of WiFi which solved it. I suppose the problem was slow or failing and unstable WiFi connection, because the problem arrised mainly with larger video files.