The instance doesn't provide anymore information beyond what's shown in this screenshot, it more or less prints exactly this in the logs too the moment it occurs.

Mod List
GeckoLib 4 [4.8.2]
Hybrid Aquatic [1.5.1]
Kotlin for Forge [5.10.0]
Lithostitched [1.5.0]

Minecraft Version: 1.21.1
Neoforge Version: 21.1.211

Got my logs for this exact issue, plugged it into GPT and it basically said that it was the fishing rod bobber sending an "empty" item as a packet which caused the client to disconnect.
I'll add logs and GPT convo for reference:

Log:
message.txt

This is the convo with ChatGPT:
https://chatgpt.com/share/68f5d0d8-a04c-8007-a8cb-2029bdb5f931

This was on Modrinth Launcher
Version: NeoForge 1.21.1 (NeoForge 21.1.211)
Mods:

geckolib-neoforge-1.21.1-4.8.2.jar
[1.21.1-Neoforge] Hybrid Aquatic 1.5.1.jar
kotlinforforge-5.10.0-all.jar
lithostitched-1.5.0-neoforge-1.21.1.jar

While I'm at it, there is also a crash that happens when interacting (right clicking) with the otter
Log: https://mclo.gs/muZkaCp (Same mod configuration as above)

Fixed in 8d88f5d