Nextcloud 15 server crashes partially on Raspberry Pi 3 B+ resp. loads media very slowly

I am using Debian 9 stretch (Raspbian) and just yesterday installed Nextcloud 15 from scratch replacing my Nextcloud 13 installation on my Raspberrry Pi 3 B+ server. I even deleted the old database and made a completely new one. I followed the instruction at exactly (including changes in php.ini for php 7.2) and it looked like it would work. For test reasons i started one video from an external storage (Samba harddisk attached to my RPi 3 server) via the Nextcloud Android App on my new Nextcloud 15 installation and it started to play in a very small rectangle on my handy but broke after some seconds and after that i could not use the app anymore to view any media. Yet still i could load and browse directories on my external storage. I suppose that that means the server did not crash completely but only the media playing subsystem crashed. Does anyone know what could be the problem?

Is there any possibility to make the nextcloud server to transfer scaled and compressed images to the client. My photo directory contains highres photos from my handy cam each of about 5 MB and more. For some time i tried to use a DLNA server for accessing that media and that would compress and convert the pictures resp. videos before transferring or streaming it. But i have to admit that the miniDLNA from Debian 9 stretch i got never working satisfactorily in my network.

On my Nextcloud 13 installation it was possible to watch videos and movies at a reasonable quality on a remote device or in my home network, now it looks like that is not possible anymore.