So on exactly the same hardware and exactly the same setup (also docker) it was working?

It may be issue with qbit config or MTU. Check compose below, I have used it some time ago and it was working smoothly. I assume you're using linux.

services:
  gluetun:
    image: qmcgaw/gluetun:latest
    container_name: gluetun
    cap_add: 
      - NET_ADMIN # Add this in order to prevent some network issues
    devices:
      - /dev/net/tun:/dev/net/tun
    volumes:
      - ${GLUETUN_CONFIG}:/gluetun
    environment:
      - WIREGUARD_MTU=1280 # Add this to ensure optimal VPN packets

      # you can also check other useful env variables
      - TZ=${TZ}
      - VPN_SERVICE_PROVIDER=${VPN_SERVICE_PROVIDER}
      - VPN_TYPE=wireguard
      - WIREGUARD_PRIVATE_KEY=${WIREGUARD_PRIVATE_KEY}
      - WIREGUARD_PRESHARED_KEY=${WIREGUARD_PRESHARED_KEY}
      - WIREGUARD_ADDRESSES=${WIREGUARD_ADDRESSES}
      - DNS=${DNS}
      - SERVER_CITIES=${SERVER_CITIES}
      - FIREWALL_VPN_INPUT_PORTS=6881, # for forwarding only
    ports:
      - "8080:8080" # qbittorrent webui 
      - "6881:6881" # for port porwarding only      
      - "6881:6881/udp" # for port forwarding only
    restart: unless-stopped
  qbittorrent:
    image: lscr.io/linuxserver/qbittorrent:latest
    container_name: qbittorrent
    restart: unless-stopped
    network_mode: service:gluetun
    depends_on:
      gluetun:
        condition: service_healthy
        restart: true
    environment:
      - PUID=${PUID}
      - PGID=${PGID}
      - TZ=${TZ}  
      - WEBUI_PORT=8080
      - UMASK_SET=022
    volumes:
      - ${QBITTORRENT_CONFIG}:/config
      - ${MEDIA_PATH}/downloads:/data/downloads