This is exacltly what happen on my samsung tv app but If I remeber well it worked on the browser … tonight I will test again at home and let you know the result.
So I’m doing some testing and neither my web client "clientIdentifier"s nor either of my android plex clients work.
Always “host is unknown for machine ID xyz”.
So strange because on all clients I was able to READ whats playing, the volume, etc. but not PUT any data.
I tried adding my plex.direct server URL to my host file as mentioned in the linked thread, to no avail. Maybe I did it wrong, is this the correct way to add a custom host to /etc/hosts? :
185.123.456.210 plex.direct
Or do I need to add the full 185-123-456-210.aa9aaasdfhgdfg33333388ad8061.plex.direct address?
EDIT: Also I run my own DNS Server here at home via my RaspPi - pihole - I thought maybe that was messing up the dns query for the above server url, so I reverted to Google’s DNS (8.8.8.8), but that didn’t work either 
UPDATE:
Out of my pi-hole DNS logs:
Mar 21 21:23:01 dnsmasq[23806]: 883 127.0.0.1/34136 query[A] 185-123-456-210.aa9aaf2aa4fe454faa65ada588ad8061.plex.direct from 127.0.0.1
Mar 21 21:23:01 dnsmasq[23806]: 883 127.0.0.1/34136 forwarded 185-123-456-210.aa9aafddddddddddddfaa65ada588ad8061.plex.direct to 208.67.220.220
Mar 21 21:23:01 dnsmasq[23806]: 884 127.0.0.1/34136 query[AAAA] 185-123-456-210.aa9aafddddddddddddfaa65ada588ad8061.plex.direct from 127.0.0.1
Mar 21 21:23:01 dnsmasq[23806]: 884 127.0.0.1/34136 forwarded 185-123-456-210.aa9aafddddddddddddfaa65ada588ad8061.plex.direct to 208.67.220.220
Mar 21 21:23:01 dnsmasq[23806]: 883 127.0.0.1/34136 reply 185-123-456-210.aa9aafddddddddddddfaa65ada588ad8061.plex.direct is 185.123.456.210
Mar 21 21:23:01 dnsmasq[23806]: 884 127.0.0.1/34136 reply 185-123-456-210.aa9aafddddddddddddfaa65ada588ad8061.plex.direct is NODATA-IPv6
So in short, it doesn’t know where to forward that address itself, it looks to OpenDNS (208.67.220.220) which I have setup as my “fall-back” DNS provider on there. Both on IP4 ([A]) as well as IPV6 ([AAAA]).
It doesnt get an answer for V6, but for V4 it gets the correct IP. So it shouldddd be able to find my host!
Grrr…