Freeze system during optimizaion

[jorman_franzini_gmail_com]

Server Version#: 1.32.0.6973

Hi, I am experiencing a problem with the plex server. If I play a video from outside my home, I have no problem (I’ve only tried 20minutes), the video plays smoothly and I guess it is “converted”.
If however, I tell Plex to optimize a video, then after a while (it is not always the same) the whole system goes into freeze and I am forced to reboot the machine.
I own a Qnap TS-251B “modified” with Debian 10 as the operating system, 16Gb RAM.
The processor is a J3355 → Intel Celeron Processor J3355 2M Cache up to 2.50 GHz Product Specifications and if I understand correctly I can “take advantage” of the integrated GPU as “Intel® Quick Sync Video” is available.
Plex is run through Docker and this is the stack that runs it:

version: "3"
services:
    plex:
        image: lscr.io/linuxserver/plex:latest
        container_name: plex-server
        network_mode: host
        environment:
            - PUID=65534
            - PGID=65534
            - TZ=Europe/Rome
            - VERSION=docker
        volumes:
            - /docker-config/AppData/Config/Plex:/config
            - /data/Multimedia/Serie Tv:/tv
            - /data/Multimedia/Film:/movies
            - /data/Multimedia/Foto:/photo
        devices:
            - /dev/dri:/dev/dri
        restart: unless-stopped

and this is the startup log:

[migrations] started
[migrations] no migrations found
───────────────────────────────────────
      ██╗     ███████╗██╗ ██████╗ 
      ██║     ██╔════╝██║██╔═══██╗
      ██║     ███████╗██║██║   ██║
      ██║     ╚════██║██║██║   ██║
      ███████╗███████║██║╚██████╔╝
      ╚══════╝╚══════╝╚═╝ ╚═════╝ 
   Brought to you by linuxserver.io
───────────────────────────────────────
To support LSIO projects visit:
https://www.linuxserver.io/donate/
───────────────────────────────────────
GID/UID
───────────────────────────────────────
User UID:    65534
User GID:    65534
───────────────────────────────────────
**** Server already claimed ****
**** creating video group videoi983 with id 107 ****
**** adding /dev/dri/renderD128 to video group videoi983 with id 107 ****
**** adding /dev/dri/card0 to video group videoi983 with id 107 ****
Docker is used for versioning skip update check
[custom-init] No custom files found, skipping...
Starting Plex Media Server. . . (you can ignore the libusb_init error)
[ls.io-init] done.
Critical: libusb_init failed
The '--scan' operation is deprecated and will be removed in future versions of Plex Media Server.
The '--scan' operation is deprecated and will be removed in future versions of Plex Media Server.
The '--scan' operation is deprecated and will be removed in future versions of Plex Media Server.

I couldn’t figure out why it crashes from one moment to the next, I also can’t figure out if the GPU is actually being used for video file optimization.
Does anyone have any ideas on how I can “capture” a log to figure out what is causing the whole system to crash? Or, does anyone have any advice for this situation?

Jorman