0. ALWAYS PROVIDE INFORMATION ABOUT MODLOADER (Fabric / Forge) AND MOD VERSION (For Example 1.19.2-8.2.10)

• LWJGL 3: 3.3.3
• Minecraft: 1.20.6
• NeoForge: 20.6.135
• Traveler's Backpack: 9.6.3 Modrinth: travelersbackpack-neoforge-1.20.6-9.6.3.jar last modified 2025-06-20
• prism launcher 9.4

other mods installed:

• architectury 12.1.4
• balm 10.3.6
• cloth config v14 api 14.0.139
• clumps 17.0.0.1
• controlling 17.0.1
• curios api 8.1.0+1.20.6
• dynamic lights 1.8.7+mod
• embeddium 0.3.20+mc1.20.6
• embeddium extra 0.5.6+mc1.20.6-build.123
• inventory essentials 11.0.3
• inventory tweaks refoxed 1.20.6-1.1.1
• jade 14.2.3+neoforge
• lighty 3.0.0-beta.3
• mouse tweaks 2.26
• roughly enough items (REI) 15.0.787
• searchables 1.0.1
• tom's simple storage mod 1.6.17
• xaero's minimap 25.2.6
• xaero's world map 1.39.9

1. Describe the bug

crash/exception upon opening backpack in solo play via "b" key with the following error log message:

[22Jun2025 19:39:35.566] [Render thread/WARN] [net.minecraft.client.multiplayer.ClientCommonPacketListenerImpl/]: Client disconnected with reason: Failed to open a screen with advanced data: readerIndex(1) + length(1) exceeds writerIndex(1): UnpooledHeapByteBuf(ridx: 1, widx: 1, cap: 1/1)

minecraft window stays open, presents the error message in the minecraft window with a header "Connection lost" and button "Back to Server List", despite being in single player. clicking the button goes to server list "scanning for games on your local network".

changing the control/key for opening the backpack has no impact on the behavior.

2. Write steps to reproduce the bug

• i progressed through the game, crafted my first standard/base backpack
• filled the backpack with items. there may have been some empty slots remaining
• filled the left tank with water
• clicked the in-backpack button that put the backpack on my character's back
• now when attempting to open the backpack using the assigned key, it instantly crashes the game

additionally:

• assigning to a different key doesn't change the behavior
• opening my game to LAN play using the option "Open to LAN" has no visible effect
• i installed curios API which revealed the curios button in the inventory/character screen. i click the curios button which reveals two slots, one being backpack, but the slot appears empty.
• i used /give to get another backpack. in this backpack, i don't see the button to equip it to my back. i also cannot place this extra backpack into the curios API slot.

3. What is expected behaviour

pushing the key assigned to opening the backpack opens the backpack interface without crashing.

4. Screenshots (If applicable)

1.20.6 is not supported anymore, but I suggest to update to latest neoforge version for 1.20.6