Player doesn't work in iframed Nextcloud folder

[jbreu]

Background: I use Nextcloud to host big files (mostly videos) for a phpBB-based forum. I embed the regarding Nextcloud folder via iFrame into the forum. Until Nextcloud version 17 users could click on a video and use this app to directly get the video played inside the iFrame. During last months I updated first to Nextcloud 18 and later 19, and the video doesn't play anymore in the iFrame (I dont know exactly when it broke). However, the video player works on the same video when I open the according folder "natively" in the browser.

Expected behaviour

Clicking on a video in iframed Nextcloud folder plays according video in player.

Actual behaviour

Clicking on a video in iframed Nextcloud folder opens the player's black overlay and the spinner spins forever.

Steps to reproduce

1. Include a Nextcloud (shared) folder with videos via iFrame into another page
2. Install video app
3. Click on video in folder natively and on ifram page.

Hints:
Firefox console shows an error in Viewer.vue. This error doesnt show up when the video is opened natively (without iframe).