Getting error when using network_mode:container?

I have a vpn client container that seems to be working properly, it connects to the server and all that. I am trying to use it as my network for other containers but I am getting this error whenever I try to create the new containers:

"ERROR: for [container]  Cannot create container for service [container]: conflicting options: host and and the network mode"

I am stumped because I originally had this working at one point. The first time I tried making this container all the other ones were able to connect to it successfully, but I had to recreate it trying to fix an unrelated issue and, without changing anything, this error showed up.

I have tried making new containers under different names and completely remaking the docker-compose and conf files, but it doesn't seem like anything is fixing the issue. I am unable to find a solution that works anywhere online.

Any help would be greatly appreciated, I don't know what else to try at this point, thank you!

Edit: adding compose files

vpn container:

version: "2.1"
services:
  mullvad:
    image: linuxserver/wireguard
    container_name: mullvad
    hostname: mullvad
    cap_add:
      - NET_ADMIN
      - SYS_MODULE
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=$TZ
    volumes:
      - ./config:/config
      - /lib/modules:/lib/modules
    ports:
      - 51820:51820/udp
    sysctls:
      - net.ipv6.conf.all.disable_ipv6=0
      - net.ipv4.conf.all.src_valid_mark=1
    restart: always

networks:
  default:
    external:
      name: vpn

example container:

version: "2.1"
services:
  qbittorrent:
    image: linuxserver/qbittorrent
    container_name: qbittorrent
    hostname: qbittorrent
    network_mode: container:$VPN_CONTAINER
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=$TZ
      - UMASK_SET=022
      - WEBUI_PORT=8080
    volumes:
      - ./config:/config
      - ./downloads:/downloads
    ports:
      - 6881:6881
      - 6881:6881/udp
      - 8080:8080
    restart: unless-stopped