Not sure if this can be regarded as an issue or not but I'll describe the symptoms and will be interested in the comments.

I have a 1.21.10 world and have been playing on a server modded with FallingTree using vanilla clients. As 1.21.11 is out, I wanted to upgrade the world so I installed forge 1.21.11 on the server and copied the world over (I maintain a new directory for each version). Upon trying to access the server with a vanilla client, I get a Network Protocol Error and get kicked off.

However, if I try to access the server with a vanilla client and allow the server to create a new world, no error. If I use a forge client with the old, no error. It seems to be the specific combination of using an existing world and the vanilla client.

For my purposes right now, I'm fine with using Forge clients. It would be nice to be able to use vanilla (since the mod I use doesn't require client modding). Just wondering if this is in any way possible.

Note that I tried removing FallingTree just in case and it made no difference.

The logs show

Time: 2025-12-12 11:54:23
Description: Packet handling error

java.lang.ArrayIndexOutOfBoundsException: Index 18 out of bounds for length 18
    at ama.a(SourceFile:114)
    at hig.a(SourceFile:665)
    at agp.a(SourceFile:54)
    at agp.a(SourceFile:12)
    at hig.a(SourceFile:2377)
    at adm.a(SourceFile:19)
    at adm.a(SourceFile:7)
    at xl$a.a(SourceFile:55)
    at xl.b(SourceFile:38)
    at gfj.d(SourceFile:1337)
    at gfj.j(SourceFile:966)
    at net.minecraft.client.main.Main.main(SourceFile:250)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
    at ama.a(SourceFile:114)
    at hig.a(SourceFile:665)
    at agp.a(SourceFile:54)
    at agp.a(SourceFile:12)

-- Incoming Packet --
Details:
    Type: clientbound/minecraft:bundle
    Is Terminal: false
    Is Skippable: false
Stacktrace:
    at abb.a(SourceFile:41)
    at hia.a(SourceFile:146)
    at hia.a(SourceFile:125)
    at xl$a.a(SourceFile:60)
    at xl.b(SourceFile:38)
    at gfj.d(SourceFile:1337)
    at gfj.j(SourceFile:966)
    at net.minecraft.client.main.Main.main(SourceFile:250)

-- Connection --
Details:
    Protocol: play
    Flow: CLIENTBOUND
    Is Local: false
    Server type: OTHER
    Server brand: forge

-- System Details --
Details:
    Minecraft Version: 1.21.11
    Minecraft Version ID: 1.21.11
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 21.0.7, Microsoft
    Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Microsoft
[redacted]

Edited December 14, 2025Dec 14 by Richy_T

So... ummmm, forge "1.2.11"? Do you mean forge "1.21.11" (as said in your crash report)? If you really meant what you said, then you're really gonna' have to provide a download link here (and maybe even a time machine). Also, please specify the exact version of forge that you are using. Because your crash report is completely obfuscated (for whatever reason), people are going to need access to the mappings in order to make sense of this crash report. Provide all of this (the mappings should be easily accessible if you provide the exact forge version), and I will edit this crash report to make much more sense and post it back here via pastebin. Hopefully then it will be much easier for others to answer your question.

I will say though, just based on the error message I can see, chances are likely slim that you can use a vanilla client. These reasons would take a little while to get into, but even if chances look slim, you shouldn't let this discourage you; you never really know if a problem like this can't be solved unless we try.

Hope this can help us get somewhere easier to work with.

Let me know!

Yes. 1.21.11. Unfortunately, the edit button is not working for me.

(Never mind, the edit link in the ... works)

If I can't use vanilla, that's fine. I'd be curious to know why as it was working for 1.21.10 but ultimately, I won't complain because the mods are handy. Moreso just reporting to possibly help others.

Just to be definitive though, is unmodded forge supposed to be compatible with vanilla or is that basically unsupported?

Edited December 14, 2025Dec 14 by Richy_T

I do not believe that forge is compatible with vanilla, even if it is un-modded. This is mainly due to forge having different kinds of network packets as well as its own required client and server network handler that vanilla minecraft just doesn't have (although this networking should be done on a different named channel that forge's packets wont be being sent to if on vanilla). Forge might also add additional data onto some vanilla packets in some versions (which is just speculation, as I have never seen evidence of this before during my time modding, so it could be unlikely). There could be other reasons, but I would have to do some testing to find out exactly what is going on. In the meantime, I will try and find the right mappings to make your log file more readable, which should help get to the bottom of your specific issue here.

I will get back to you.

Edit: I will get back to you... In a while: this is going to be a bit of an undertaking.

Edited December 14, 2025Dec 14 by Jackson Perr
Just adding more info without having to make another comment

Well, that wasn't so bad! I added function parameters to each of the methods to make it easier to understand which function is which. Hope this helps out!

Pastebin

Crash Log - Pastebin.com

Pastebin.com is the number one paste tool since 2002. Pastebin is a website where you can store text online for a set period of time.

Forge should be completely compatible with vanilla clients, and vanilla servers.

The issue seems to be that your old entities contain a Attribute at an ID that the vanilla client doesnt like.

As I asked on reddit, you should share your old save file, as well as the mod list the server was using before and after the update.

Give us information to debug and we could probably find the issue.

Sure thing. My world is just under 1GB zipped. Is there a preferred transfer method?

I would say that you should just do whatever works... without giving somebody a virus. The only goal is to get the files from your computer to other people, right? Shouldn't really matter too much the median. At least I don't think.