Grafana graph as Image in Sitemap only sporadically loading

  • Platform information:
    • Hardware: virtualized as Proxmox LXC, 4 CPU cores, 2GiB RAM.
    • OS: Debian 10.13
    • Java Runtime Environment: openjdk 11.0.17 2022-10-18 LTS
    • openHAB version: 3.3

I’m trying to show a grafana graph of the current power my new solar panel produces in my sitemap.
The important line in the sitemap is:

Image refresh=60000 url="http://grafana.mydomain.local:3000/render/d-solo/6eWzvNN4z/dashboardname?orgId=1&${__url_time_range}&panelId=2&width=800&height=300"

opening this URL in the browser of the client works perfectly. In the openhab app on my phone it rarely shows up, but most of the time immediately disappears after flashing an “image-loading”-icon, where the graph is supposed to go.

there is no error in /var/log/openhab/openhab.log

I also tried Webview instead of Image, with similar results.

Does anybody have a hint what I am doing wrong here?

best regards,