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 1.2.10 world and have been playing on a server modded with FallingTree using vanilla clients. As 1.2.11 is out, I wanted to upgrade the world so I installed forge 1.2.11 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]

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!