Videostream blank while not in home network or connected via VPN

Good morning,
I’ve got a strange behaviour with the video of my webcam.
Opening the sitemap in the browser shows the image no matter if I’m in my home network, connected via vpn or accessing over the public URL.

Using the openhab iOS-App I only see the image if I’m in my home network or connected via VPN.
Trying to access it over the public URL (everything else, including realtime refresh of MQTT-Items works) I only see blank.

This is the entry in the sitemap.

Image item=WCAM01_ImageURL refresh=500 visibility=[WCAM01Activation==ON]

Can anyone help? Thank you!