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Minecraft and Mod version 🐞
Tested across 1.20.4, to 1.21, including Fabric, Forge, and NeoForge launchers.

Description of the crash 💥
Creating an NPC in 1.20.4 Forge, and porting to 1.21 Fabric, results in the following error, alongside a client side disconnect from the server in a multiplayer setting: https://pastebin.com/evCXtjtx
Creating an NPC in 1.21 NeoForge and porting to 1.21 Fabric results in the exact same error.
Loading up EasyNPCs in Forge on 1.21 with latest release results in the following crash: https://pastebin.com/d04jXdmu

Steps to reproduce the crash ⚙️

1. Create NPC on 1.20.4 Forge/1.21 NeoForge
2. Export and load on Fabric 1.21
3. Interact, client sided crash.

Expected behavior before the crash ✔️
With the multiple types of NPCs, actions and dialogues tested, including down to a simple "if this dialog pops up, test successful" and all failing, the desired result is just simply either the dialog, or the action running on interact instead of the crash :P

Logs 📝
Please provide the following logs:

crash-2024-07-30_05.12.52-server.txt
The server isn't producing a debug.log, which I'm entirely sure is user error.

Additional context 📋
I should note that all of these NPCs worked on their "home" loaders, before being exported to Fabric.

Thanks for the report. Will check this more in detail.

Relevant part:

Caused by: java.lang.ClassCastException: class de.markusbordihn.easynpc.network.message.client.OpenMenuCallbackMessage cannot be cast to class net.minecraft.class_8711 (de.markusbordihn.easynpc.network.message.client.OpenMenuCallbackMessage and net.minecraft.class_8711 are in unnamed module of loader net.fabricmc.loader.impl.launch.knot.KnotClassLoader @7ca48474)
	at net.minecraft.class_9139$6.encode(class_9139.java:42) ~[server-intermediary.jar:?]
	at net.minecraft.class_8710$1.method_56489(class_8710.java:52) ~[server-intermediary.jar:?]
	at net.minecraft.class_8710$1.method_56490(class_8710.java:57) ~[server-intermediary.jar:?]
	at net.minecraft.class_8710$1.encode(class_8710.java:39) ~[server-intermediary.jar:?]
	at net.minecraft.class_9139$8.encode(class_9139.java:81) ~[server-intermediary.jar:?]
	at net.minecraft.class_9139$9.method_56442(class_9139.java:97) ~[server-intermediary.jar:?]
	at net.minecraft.class_9139$9.encode(class_9139.java:87) ~[server-intermediary.jar:?]
	at net.minecraft.class_9136.method_56426(class_9136.java:53) ~[server-intermediary.jar:?]
	... 37 more
[22:19:47] [Server thread/INFO]: xenocream lost connection: Internal Exception: io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (easy_npc:open_menu_callback_message)

Thanks for the report, this should be fixed with 5.4.1.