[Bug] Picking up a player disconnects every player on server, io.netty.handler.codec.EncoderException

[FoxySophie]

Describe the bug
When picking up a player in my modpack sometimes it will disconnect every player on the server.

To Reproduce
Steps to reproduce the behavior:

1. Pick up a player
2. ???
3. Everyone disconnets

I can't reliably reproduce the issue right now since I don't want to bother my players, but I will attach the log of my client and the server and once it's just me and a friend we will try to find a way to reproduce it.

Game Information (please complete the following information):

• Minecraft Version: 1.21.1
• Mod Loader: Fabric 0.18.4
• Carry On Version: 2.2.4

Additional context, logs
This is in a custom modpack. I don't want to attach the full logs since it contains player names and chat messages, but I will include the snippets of the crash.
The modpack can be found here: https://modrinth.com/modpack/serene-pioneers-3

Disconnect message on the clients:
[15:10:25] [Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (carryon:sync_carry_data)

server_latest.log

[snekaholic]

I can second this happening on the latest version for NeoForge as well. Same error message, sometimes happens when picking up barrels as well (oddly enough it is really inconsistent. Some sessions it will happen immediately, others you can use CarryOn just fine).

This is on a server with custom modpack as well

[abb3v]

+1 having the same issue. For me specifically it happens sometimes to do with boats:

1. put an entity into a boat.
2. put a player ontop of that entity.
3. remove the player.

It will either crash when i pick up the player, or place them.

[Akeoott]

Having the same issue here! The moment i place a player it crashes.
My Modpack is minimal and performance focused so i doubt its a compat issue.

I'm on Fabric 1.21.1

The disconnect message is the same as presented in the issue.
I can reproduce the issue. Just pick up a player and put that player down. Boom, crash.

[abb3v]

@Akeoott Apparently the solution is to downgrade to 2.2.2 I believe(?), have not tested yet!

[snekaholic]

I can confirm downgrading to the previous (and only other) version on 1.21.1 NeoForge fixes the issue

[IdrisQe]

Thank you for the info, this'll be helpful! It'd be nice if a fix got released, or the bugged version got pulled from platforms

[francoismonteil]

I can reproduce what looks like the same issue on a Fabric 1.21.1 multiplayer server.

Environment:

• Minecraft 1.21.1
• Fabric Loader 0.18.4
• Fabric API 0.116.8+1.21.1
• Carry On 2.2.4.4
• Cobblemon 1.7.3+1.21.1
• Dedicated multiplayer server

Observed behavior:

• When this bug happens, every online player gets disconnected, not only the player who triggered it.
• I reproduced it multiple times on 2026-03-12 at 21:07, 21:50, 22:03, 22:05 and 22:07 (server local time, UTC+1).
• I now have a clearer trigger:
  • around 22:02, player A was being carried by player B, and when player B crouched, 3 players were disconnected
  • around 22:06, another player was carrying me, and when the carrier crouched, both of us were disconnected, but a third player in The End was not disconnected
• So the trigger appears to be: a player is carrying another player, then the carrier crouches.
• Based on these two reproductions, the disconnect does not look strictly global to the whole server. It seems to affect players in the same relevant world/session context, while a player in another dimension was unaffected in one repro.
• Server logs always show the same payload id:

[22:03:11] [Netty Epoll Server IO #15/ERROR]: Error sending packet clientbound/minecraft:custom_payload
io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (carryon:sync_carry_data)
Caused by: java.lang.NullPointerException

• Client logs show:

[22:03:13] [Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (carryon:sync_carry_data)

• I also saw secondary server-side effects such as:
  • Negative index in crash report handler
  • Failed to save player data for <player>

What I can confirm:

• this is reproducible
• it happens in multiplayer
• it disconnects multiple players at once
• the failing payload is always carryon:sync_carry_data

What I cannot confirm yet:

• I have not reduced this to Carry On without Cobblemon
• I have not reduced this to a minimal mod set yet

So for now I can only report this as a Carry On <-> Cobblemon multiplayer compatibility issue, but the payload and disconnect behavior look very close to this issue.

[nirkyrion]

Same issue on server 1.21.1 Neoforge 21.1.221, carry on 2.2.4 https://mclo.gs/PQ8hcQw

[Immodial]

Confirming this happens for me too, and was solved by downgrading. Huge thanks @abb3v !!

[reasure3]

I have experienced a similar problem. The same symptoms appear not only when moving players but also when moving entities such as villagers (may be entities that have custom data such as inventory?).

My version is:
Minecraft 1.21.1
Neoforge 21.1.227
Carry On 2.2.4

My guess is that there is no issue if there is only one player on the server, but the problem seems to occur if multiple players are nearby.

[blitzyuk]

I'm having a similar problem but it happens when placing down spawners. Everyone on server gets disconnected with the Internal Exception error.

Minecraft 1.21.1
Neoforge 21.1.228
Carry On 2.2.4.4

Strange how in the changelog for this version it has "Fixed bugs related to player carrying"

[Skop12]

Same problem and error, but it just started happening. Was fine before.

MC 1.21.1
Neoforge 21.1.227
Carry On 2.2.4

Will try downgrading