Plex VPN Iphone App Problem

Hello everyone, maybe someone can help. I have a Plex server on a TruenasCom 25 and a Wireguard VPN server. Both are running well.

When I open the IP of the Plex server via VPN with my phone browser, everything works fine as usual. But the Plex app can't find any libraries. What could be the reason for this?

Best regards