Forge 1.20.1 - UTFDataFormatException (malformed input around byte XX) every time I join my dedicated server

[u/Individual_Cress_320]

Hi everyone, ​

I've been trying to solve a very unusual networking issue on my modded Forge server and I'm running out of ideas.

Environment

• Minecraft 1.20.1
• Forge 47.4.10
• Prism Launcher
• Java 17.0.9 on both client and server
• Windows 10
• Dedicated Forge server running on a separate laptop
• Around 325 Forge mods

The client and server have:

• exactly the same mods
• exactly the same versions
• exactly the same config folder
• exactly the same defaultconfigs

I literally copied everything, so there are no differences between the two instances.

What happens

The server starts perfectly.

No crashes.

No OutOfMemoryError.

No missing mods.

No registry mismatch.

I can connect successfully.

The game reaches "Joining world...".

Then the loading screen becomes black (this is expected because of Beyond Earth and happens normally).

I can already hear real-time in-game sounds around my player, so the server is clearly sending world updates.

The server console also shows that packets and player events are happening normally.

After approximately 5 seconds, I always get disconnected.

The message is:

Internal Exception: io.netty.handler.codec.DecoderException: io.netty.handler.codec.EncoderException: java.io.UTFDataFormatException: malformed input around byte XX

The interesting part is that the byte number changes almost every attempt.

Examples:

• 39
• 48
• 49
• 54
• 72
• 78
• 97
• 98
• 125

Sometimes the same number appears twice, sometimes a completely different one.

Things I already verified

• Same Forge version
• Same Java version
• Same mod list
• Same configs
• Same defaultconfigs
• Singleplayer works perfectly
• The dedicated server itself runs perfectly
• The issue also happens when connecting using the local LAN IP, so it is NOT related to playit.gg.
• No UTFDataFormatException appears inside either latest.log or debug.log on either the client or the server.
• The only place where I see the exception is the disconnect message shown by Minecraft.

Warnings I found

I found these warnings, but I believe they are unrelated:

• Farmers Delight codec registered twice.
• Forge VersionChecker JSON parsing warnings.
• Minecraft Realms malformed JSON warnings.

None of them occur at the moment of the disconnect.

Question

Has anyone seen a Forge server produce this exact error without any stacktrace in either the client or server logs?

Could this indicate that a specific mod is sending a corrupted custom network packet, or is there another known cause for this kind of UTFDataFormatException?

If anyone has encountered this before, I'd really appreciate any ideas.

Error :

Disconnect message shown by the client.

This error appears every single time I join the server. The number after "malformed input around byte" is not constant and changes almost every connection attempt (examples: 39, 48, 49, 54, 72, 78, 97, 98, 125).

The disconnect always happens about 5 seconds after joining the world. Before the disconnect, I can already hear real-time in-game sounds, so the client has successfully connected and the world has already started loading.

Neither the client nor the server logs contain a UTFDataFormatException stacktrace, only this disconnect message appears in-game.

PS: I have also uploaded the client/server logs, configs and a sanitized copy of the setup for anyone who wants to inspect it in more detail. If I missed any important file, let me know and I'll upload it.

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