Complete Camera Roll Upload Stops Constantly

I have ~9000 photos/videos in my iPhone to upload. The complete upload function manages to upload ~1000, after that the upload stops.
The app still shows the missing ~8000 files to upload but nothing happens.

When I switch the app and interact with another (to likely cause the system to pause the Nextcloud app) 5 files get uploaded and then again the upload stops. Just closing and restarting the nextcloud app is not helping, I need to do something to trigger this 5 file upload.

This behavior is reproducible (reinstalling the nextcloud app, changing settings like maximum compatibility or the upload folder).

Needless to say, this is annoying because the complete upload is therefore useless for me.

I am using a fresh installed nextcloud 19.0.1 server, Ubuntu 20.04, in wsl. And the newest iOS app (I have the described problem also with the previous version).

PS: Server and app log file does not show any errors.