Problem with Chromecast, Android and Plex.
I see intermittent problems when casting via Android to a Chromecast. This is when I'm remote from my QNAP.
The Plex server has access from outside, and is running 'secure'.
I've sometimes been able to solve it by uninstaling the Plex App from the Android device, and then reinstall it.
Other times that hasn't helped.
If I start plex in a browser, it will cast from that without much fuss, but it's not always I have a wireless pc device at hand.
The Android problem has been shown both with Android 7.1 and Android 8.0.