Impossible to join a server with "Essential Mod"

[NoltoxGit]

Expected Behavior

I expected to be able to join the server through Velocity while using the Essential Mod (https://modrinth.com/mod/essential/version/YOds1odx) on Minecraft 1.21.4 (Fabric 0.16.3), just like with any regular client.

Actual Behavior

When attempting to connect to the proxy using the Essential Mod, the client is immediately disconnected with the following error message:

Une erreur interne s'est produite lors de votre connexion.

Velocity logs also show this disconnect with no detailed explanation.

Steps to Reproduce

1. Set up a Velocity proxy with online-mode = true.
2. Connect a backend Paper server running regular plugins (listed below).
3. Start the game using Minecraft 1.21.4 + Fabric Loader 0.16.3 with the Essential Mod installed from Modrinth.
4. Attempt to connect to the server through Velocity.
5. Observe that the player is disconnected instantly with an internal error.

Plugin List

Proxy (Velocity):

• velocity (no additional plugins installed)

Backend (Paper):

• nightcore
• ajParkour, ajQueue, AzLink, CMI, CMILib, CoinsEngine, DeluxeMenus, DiscordSRV, EconomyBridge, FastAsyncWorldEdit
• FreedomChat, ImageFrame, ItemJoin, LibsDisguises, LiteBans, LobbyGames, LuckPerms, mc-PlayerVisibility, mclogs, MythicMobs
• packetevents, Pl-Hide-Pro, PlaceholderAPI, PremiumVanish, ProtocolLib, TAB, Vault, ViaVersion, WorldGuard

Velocity Version

Velocity 3.4.0-SNAPSHOT (git-74d05211-b494)

Additional Information

Velocity configs: https://mclo.gs/AU2HB1Z

• The issue occurs even without any plugins installed, on a fresh proxy setup.
• The backend server has not been tested without Velocity, so I’m unsure if this is a Velocity-specific issue.
• Full logs:
  • Velocity logs: https://mclo.gs/J8LWCHe
  • Minecraft client logs: https://mclo.gs/zhbx7vB
• I suspect a possible incompatibility between Essential’s networking layer and how Velocity handles connections.

Would appreciate if this could be investigated — thanks a lot! 😊

[CC-Odin]

Netty 4.2.0 in Velocity build 491 appears likely to be what breaks this. No idea why or how it's breaking, but downgrading to build 490 of Velocity will allow Essential Mod to immediately begin working again as expected.

[GamingGeek]

Essential support agent here, this is due to the issue I reported in #1550. Builds 492 and earlier should work as the issue was introduced in build 493, not the netty update in 491