My Emby server is Version 220.127.116.11 on a Mac Mini. I keep Emby player open on the Mac Mini and remote control the player using the player app from various other devices -- my Android phone, and Android tablet, my Macbook, my wife's PC laptop.
About once a week or so, when I click the "Play On" icon on a remote player, nothing shows in the list of available players to control. It just says "Play On" with nothing below it. If this happens on, for example, my phone, I'll go to my MacBook and try from there but get the same result. In other words, it affects all players on all devices when it happens. Library content from the server is still visible and playable locally on that device.
The way I've found to "fix" it is to VNC into the Mac Mini (it's headless) and change the focus of the active apps -- if Emby player is the front-most active window, I'll click on the desktop to make the Finder active, then click Emby to make it active again. If Emby isn't the front-most window, I'll bring it to the front. Basically any change to the focus of Emby player on the server fixes it.