I found it!
It’s a bug in plex media server for sure.
Under Kubernetes you need to use fully qualified domain names with at least two dots.
So: plex.tv. and not plex.tv
The second one will fail. Curl worked, because it actually translates the request into plex.tv.
The fix, for me was to use kustomize with the plex helm to add this to the statefulset:
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: plex-media-server
spec:
serviceName: plex-media-server
template:
spec:
dnsConfig:
options:
- name: ndots
value: "1"
Yes, it’s the ndots problem: Kubernetes pods /etc/resolv.conf ndots:5 option and why it may negatively affect your application performances
So the good solution is to make plex media server lookup plex.tv. or to add this yaml to the helm chart config.