[Compatibility] Strange error on dedicated servers

[muon-rw]

When sneaking on a dedicated server in certain circumstances, the player gets disconnected, and can not reconnect.

The error in the client logs is:

[14Feb2025 21:46:10.483] [Render thread/ERROR] [net.minecraft.client.multiplayer.ClientCommonPacketListenerImpl/]: Failed to handle packet ClientboundSetEntityDataPacket[id=490, packedItems=[DataValue[id=0, serializer=net.minecraft.network.syncher.EntityDataSerializer$$Lambda/0x0000000800e6b8e0@7a0f4067, value=2], DataValue[id=23, serializer=net.minecraft.network.syncher.EntityDataSerializers$3@69d394cc, value={keyPressed:1b,selected:0,tick:546,type:"INVALID"}]]]

java.lang.IllegalStateException: Invalid entity data item type for field 23 on entity LocalPlayer['Suppressed'/490, l='ClientLevel', x=-1885.81, y=230.00, z=-791.11]: old=(class java.lang.String), new={keyPressed:1b,selected:0,tick:546,type:"INVALID"}(class net.minecraft.nbt.CompoundTag)

It also outputs a network protocol error in the client debug folder with related info:
https://mclo.gs/1VaoXzD

---

What's especially odd here is that the issue is reproduced consistently in the right circumstances (every attempt to press the key results in a disconnect), but that those circumstances are incredibly unclear.

• This began happening recently in a v13 test build of our modpack https://www.curseforge.com/minecraft/modpacks/medieval-mc-mmc5, but the issue did not occur on versions v12 or earlier
• Carry On did not change versions between v12 and v13
• The issue occurs on both Neoforge 21.1.115 (v12) and 21.1.119 (v13)
• The issue does not seem to be due to any added mods between v12 and v13
• Removing the following mods prevents the crash, but adding a few of any of them reintroduces the crash:

not_enough_glyphs
discerning_the_eldritch
undergardendelight
too_many_bows
aether_protect_your_moa
chimericlib
athenaeum
arsdelight
ars_nouveau
hackersandsladhers
rpg_companions_tiny_dragons
ars_additions
gtbcs_spell_lib
gametechbcs_spellbooks
alshanex_familiars
supplementaries
amendments

• Strangest of all, adding these mods in a standalone instance with Carry On does not cause the crash.

---

I realize this probably isn't very actionable, but leaving it here as a working issue in case others encounter it and have more useful information.

[wadiv76]

I've been seeing this same error in a modified version of MMC5 and have been spending the last few days trying to identify the cause. It started after I added sophisticated backpacks. Some other tests I did were to remove all the fabric api related mods that MMC5 supports. That has seemed to help. I will be removing Carry On for now. If the problem returns I'll report back. I apologize that these details may seem like reading tea leaves, but to echo muon-rw's title, this is a strange/weird issue. I was able to trigger this disconnect error when in the game and pressing the left shift key to sneak

[WimpieRatte]

Thanks for the above post, wadiv76.
I was also getting this issue after having both sophisticated storage and backpacks and carry on.
Also only crashed our one world where we already had backpacks. When starting a new world, we could join without issues.

[Tschipp]

Hey, I'm not quite sure what this is. In the logs i see ModernFix mentioned. Can you confirm that this issue also happens without ModernFix?

[IOplus]

I'm having the same issue. I'm getting this error in Minecraft 1.21.1, using the most recent version of NeoForge for that version.

[03May2025 02:04:28.924] [Render thread/ERROR] [netprodis/]: Failed to handle packet ClientboundSetEntityDataPacket[id=136, packedItems=[DataValue[id=0, serializer=net.minecraft.network.syncher.EntityDataSerializer$$Lambda/0x00007febf8daf970@5d0f3, value=0], DataValue[id=11, serializer=net.minecraft.network.syncher.EntityDataSerializer$$Lambda/0x00007febf8daf970@79180ca5, value=[net.minecraft.core.particles.ColorParticleOption@1132a57]], DataValue[id=24, serializer=net.minecraft.network.syncher.EntityDataSerializers$3@6b398c99, value={block:{Name:"minecraft:chest",Properties:{facing:"east",type:"single",waterlogged:"false"}},keyPressed:0b,selected:1,tick:308,tile:{Items:[],id:"minecraft:chest"},type:"BLOCK"}]]]
java.lang.IllegalStateException: Invalid entity data item type for field 24 on entity LocalPlayer['IOplus'/136, l='ClientLevel', x=349.90, y=145.94, z=265.87]: old=(class java.lang.String), new={block:{Name:"minecraft:chest",Properties:{facing:"east",type:"single",waterlogged:"false"}},keyPressed:0b,selected:1,tick:308,tile:{Items:[],id:"minecraft:chest"},type:"BLOCK"}(class net.minecraft.nbt.CompoundTag)
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.network.syncher.SynchedEntityData.assignValue(SynchedEntityData.java:125) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.network.syncher.SynchedEntityData.assignValues(SynchedEntityData.java:115) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.client.multiplayer.ClientPacketListener.handleSetEntityData(ClientPacketListener.java:538) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.handle(ClientboundSetEntityDataPacket.java:54) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.network.protocol.game.ClientboundSetEntityDataPacket.handle(ClientboundSetEntityDataPacket.java:12) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:27) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:148) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:23) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:122) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.util.thread.BlockableEventLoop.runAllTasks(BlockableEventLoop.java:111) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.client.Minecraft.runTick(Minecraft.java:1155) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.client.Minecraft.run(Minecraft.java:807) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at TRANSFORMER/minecraft@1.21.1/net.minecraft.client.main.Main.main(Main.java:230) ~[client-1.21.1-20240808.144430-srg.jar%23486!/:?]
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
	at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
	at MC-BOOTSTRAP/fml_loader@4.0.38/net.neoforged.fml.loading.targets.CommonLaunchHandler.runTarget(CommonLaunchHandler.java:136) ~[loader-4.0.38.jar%23102!/:4.0]
	at MC-BOOTSTRAP/fml_loader@4.0.38/net.neoforged.fml.loading.targets.CommonLaunchHandler.clientService(CommonLaunchHandler.java:124) ~[loader-4.0.38.jar%23102!/:4.0]
	at MC-BOOTSTRAP/fml_loader@4.0.38/net.neoforged.fml.loading.targets.CommonClientLaunchHandler.runService(CommonClientLaunchHandler.java:32) ~[loader-4.0.38.jar%23102!/:4.0]
	at MC-BOOTSTRAP/fml_loader@4.0.38/net.neoforged.fml.loading.targets.CommonLaunchHandler.lambda$launchService$4(CommonLaunchHandler.java:118) ~[loader-4.0.38.jar%23102!/:4.0]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:30) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.Launcher.run(Launcher.java:103) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.Launcher.main(Launcher.java:74) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26) [modlauncher-11.0.4.jar%23157!/:?]
	at MC-BOOTSTRAP/cpw.mods.modlauncher@11.0.4/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23) [modlauncher-11.0.4.jar%23157!/:?]
	at cpw.mods.bootstraplauncher@2.0.2/cpw.mods.bootstraplauncher.BootstrapLauncher.run(BootstrapLauncher.java:210) [bootstraplauncher-2.0.2.jar:?]
	at cpw.mods.bootstraplauncher@2.0.2/cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:69) [bootstraplauncher-2.0.2.jar:?]
[03May2025 02:04:29.212] [Render thread/WARN] [netprodis/]: Strict error handling was triggered, but disconnection was prevented

I'm not getting disconnected from the server though because I have the Network Protocol Disconnect mod installed. Instead, when carrying a mob/chest like entity, the player's arms remain to their side as if they aren't holding anything, and the entity just disappears. The only way to know that you're carrying something is the camera/weight effect, and the fact that the entity reappears when you drop it, other than that, there's no animation whatsoever.

My modpack has 300+ mods, so I have no idea of what could be the problem, but I've tried removing the mods mentioned in this thread without any luck (including modernfix). For now I'll just remove Carry On from my modpack and hope that it gets fixed in the future...

[VaOnechezz]

I got this problem rn

neoforge 21.1.170
minecraft 1.21.1
Carry On 2.2.2.11 (the most fresh for 1.21.1 and neoforge)

I made modpack with this mod and now I'm getting softlocked anytime I press Shift
BUT this mod was working absolutely fine with previous version of my modpack

What server console says
[19:48:56] [Server thread/INFO] [minecraft/ServerGamePacketListenerImpl]: baraban lost connection: Disconnected
and it creates file like ones below

All files are kinda same and it's about INVALID type of data
disconnect-2025-06-13_19.47.28-client.txt
disconnect-2025-06-13_19.48.55-client.txt
disconnect-2025-06-13_19.57.29-client.txt
disconnect-2025-06-13_20.05.18-client.txt
disconnect-2025-06-13_21.32.48-client.txt
disconnect-2025-06-13_21.44.28-client.txt

This is from one of files above
java.lang.IllegalStateException: Invalid entity data item type for field 24 on entity LocalPlayer: old=(class java.lang.String), new={keyPressed:1b,selected:3,tick:0,type:"INVALID"} (class net.minecraft.nbt.CompoundTag)
This error occurs during packet handling of:
clientbound/minecraft:set_entity_data

After inspecting the player's playerdata/.dat file (using a NBT editor), I found the following structure under what appears to be field 24:

CarryOnData: {
  keyPressed: 1b,
  selected: 3,
  tick: 0,
  type: "INVALID"
}

This matches the exact structure and values from the crash report (CompoundTag with the same keys).

The key issue is:
The client expects field 24 to be a String, but the server is sending a CompoundTag.
This leads to an unhandled exception on the client when trying to assign the new value to the SynchedEntityData.

I was investigating through chatGPT and little of own research, so there might be some mistakes

and yeah, I have modernfix, but as I said it wasn't a problem with previous version of modpack that also had this (its not straight problem at least)

[VaOnechezz]

I would rename this issue as Disconnecting from dedicated server when Sneaking/pressing Shift
I haven't found this thread instantly

[Mxxstro]

It's been quite a while since this problem came to light, I'm also having problems troubleshooting it. Has anyone found a fix to it yet?

[muon-rw]

Likely the cause of ZsoltMolnarrr/BetterCombat#524

Probably resolved by addressing unsafe tracked data registration via mixin:
#829

Better Combat should probably make the same change

[edgarogh]

This issue seems to describe a possible cause: AlexModGuy/AlexsCaves#110
To quote them:

Entity Data Accessors should only be ever set by the creator of the entity. Never added to by other mods

This is very important because Entity Data Accessors is order dependent so if multiple mods tries to add these entity data accessors to mobs and on client, they run in a different order than the server did (because mod load order is not guaranteed), the client will break connecting to server.