The VPN catches all network traffic and puts it far away - you can’t be on vpn and see local network resources (casting targets) at the same time.

If your vpn has an app, check your settings for something like “local network access”.

Otherwise, start reading about split-tunnels and/or default gateways

Vpns and casting create a complicated network situation sometimes. Without being able to see exactly what was going on, it can be difficult to tell what's happening.

Sometimes casting involves sending the data from the controller, your laptop, to the renderer, your TV. That means you laptop pulls the data down and then forwards it.
Sometimes it involves telling the renderer how to get the data so that it can pull it down and play it.

When you use a VPN, you're sending your traffic through a tunnel so that it's "outside" your local network. There will be some exceptions for certain local behavior that needs to be local, unless you configured it not to but you probably didn't.

It's honestly curious that it ever works, since the VPN should make it so you're basically "not there", and so casting shouldn't be possible.

My recommendation would be to use Plex if it works.
Download on the VPN and then drop off to cast it is an alternative.

There's enough moving parts that you're not going to have any fun figuring it out, and the answer will probably be something you can't fix.

Why not use Stremio with Realdebrid, then you don't need a vpn :)

I can't offer help on the issue, but as far as I understand there are no legitimate and safe versions of popcorn time anymore. The main version got taken down a few years ago iirc. There's still plenty of unsafe versions though. Might want to double check you got it from a safe source.

The lowtech option is to connect your device physically. I do this in hotels and at home. USB C to HDMI output. Connection cables are relatively cheap, and easily long enough for most use cases.

Are you using the VPN on your laptop or phone only? If so, maybe try to run your VPN at the router level and see if you can cast then? It might work..

Isn't Popcorn time dead these days?

use stremio

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.