Same here on Android (OnePlus 12, latest version of the app) and Google Chromecast. I can successfully connect to Google Chromecast - the Plex logo is shown on the TV, but playing content doesn’t start a thing. I have the same screens on my mobile as the user who posted this:
Casting to the same device used to work fine several weeks ago.
Edit: I solved it by downgrading the Android app using these instructions:
iOS users can downgrade as well, as kaelaria mentioned in the current topic.
I did get a notice when starting playback, if I would want to allow an unsafe connection to my NAS, which I accepted. It then started playing fine on the Google Chromecast / TV.
@elan Could my last paragraph be the reason for so many users not able to cast anymore with the recent version of the mobile app? Is that security message somehow now hidden and therefore playback blocked?