MainUI popup not working when SSL is enabled on iOS device

One of my popup widgets contains an oh-webframe item which is linked to a video camera (mjpeg stream). All devices are running locally.

If I open the connection from my PC to openhab via https and the oh-webframe source is set to https, the stream is shown in the popup. Working so far as expected.

When connecting from an iOS client, the popup does not show the video anymore.

Does anybody have an idea?