No video and audio connection if running over a VPN

Hi, I hope someone can shed some light on my issue running the Talk app over a VPN.

First of all, the Talk app works fine on Nextcloud 18.0.3 together with a Coturn server and without a VPN. To reduce the amount of advertising, I want to use Blokada on my mobile phone, which creates a local VPN to block connections to specific servers.
As soon as this app is activated the Talk app can still establish a connection to my server but fails to establish an audio or video connection. Can someone please help me to understand the root cause of the problem? Is this a general WebRTC problem? I’ve already ask aunt G. for help, but couldn’t find a matching answer.

Never heard of that app. Have you checked to make sure that it isn’t blocking any of the ports used for stun, turn, and nextcloud, and that domains are properly whitelisted?

Yes, from what I can see no Nextcloud/Talk related traffic is blocked at all.