The new app uses a different method for casting, as the old will not work with new codebase of app. We set up our own proxy to it https://cast-config.plex.tv/api/v2/apps/vzb3278371473/remote/android early on so folks did not have to whitelist that other one. Not sure how strict your blocker is but maybe blocking that too.
if you are willing to share the app logs you can message them to me directly so I can make report for devs
