This is probably a ridiculous question, but I usually stream from my laptop to my LG tv or my phone to any other tv, and I find that using my VPN keeps the casting output option from working. Like in Popcorntime, the Watch Now doesn't show my tv, only the laptop app or VLC. On my daughter's fire stick I can't even cast to it unless I turn my VPN off.
I don't have any problem streaming Plex or Emby from my laptop to the tv. Is there a way to cast without compromising my privacy?
What VPN? What's its' endpoint?
Are you at home with this issue, or outside of your network?
The first thing that comes to kind is VPN usually doesn't do split-tunnel by default, so it'll consume all your traffic instead of allowing local traffic to go to the LAN with all the rest going VPN.
There may also be a filtering of services permitted through the VPN, so if it's not split-tunneling, it's trying to route everything, but blocking streaming.
I wouldn't want all my traffic going out a VPN only to come back into my LAN via a VPN connection.
I've seen similar issues with apps like Tailscale or (a long time ago) Hamachi, where the system resolves to the Mesh network IP before the local IP, routing local traffic over the VPN/Mesh instead of the LAN.
Verify your VPN has a setting to permit local traffic/connect to local network.