Reproduce:

1. Join any server with this mod
2. Just pick up something and in hotbar

Minecraft 1.20.6 Official
TerraBlender Fabric 1.20.6-3.5.0.3
fabric api: 0.97.8 1.20.6
Fabric loader: 0.15.11
JavaDungeons 2.0.1 for 1.20.6

When I try to put an item from the mod into my quick access slot from the creative, I get the error “Internal Exception: io.netty.handler.codec.DecoderException: Failed to decode packet ‘serverbound/minecraft:set_creative_mode_slot’”

Simple add mod & dependecies to server mods dir

version: '3'
services:
  minecraft-server:
    container_name: "fabric_minecraft_server"
    image: itzg/minecraft-server
    environment:
      EULA: "TRUE"
      ONLINE_MODE: "TRUE"
      TYPE: "FABRIC"
      VERSION: "1.20.6"
      DIFFICULTY: "normal"
      FABRIC_LOADER_VERSION: "0.15.11"
      ENABLE_RCON: "FALSE"
    ports:
      - "25565:25565"
    volumes:
      - .:/data
    tty: true
    stdin_open: true
    restart: unless-stopped

  restarter:
    image: docker
    volumes: ["/var/run/docker.sock:/var/run/docker.sock"]
    command: ["/bin/sh", "-c", "while true; do sleep 86400; docker restart lostwood_minecraft_server; done"]
    restart: unless-stopped

The problem was that the mod was in a subfolder, so the server wouldn't load it.

The one who coded such clear in quotes, error codes in the server and client of minecraft deserves the book “perfect programmer” from Uncle Bob.

The server does not recursively traverse all subfolders in the mods folder because of which it does not load folders.

So you can't group mods in the mods folder.