Just wanted to say thanks, I found the app responsible and force-closing it also fixes the issue. Not ideal but I reported it to the developer. 

Boot to safe mode and see if that fixes it. If so it is a 3rd party app and you should start uninstalling apps in the reverse order you installed them. 

I just get the video output symbol on VLC and the stock video player, though X-plore's video player will work. Restarting fixes the issue but only temporarily, aeroplane mode does not even though connections to anything else would be impossible then.

Model is a H870DS

My phone's convinced I'm connected to a screen and won't play video.