Disconnect: Failed to Encode Packet 'serverbound/minecraft:set_creative_mode_slot'

What the log looks like

io\.netty\.handler\.codec\.EncoderException:\s+Failed to encode packet 'serverbound/minecraft:set_creative_mode_slot'
java\.lang\.IllegalArgumentException:\s+Can't find id for 'Reference\{ResourceKey\[

How to fix it

1. Restart the Minecraft client completely after leaving one world before cheating in items in another world or server.
2. If the error triggers when clicking a favorited item (like a patterned banner or enchanted book) in Roughly Enough Items (REI), delete that favorite and re-bookmark it inside your current world.
3. Open REI's configuration screen and change the Cheat Mode method from Creative packets to the /give command, or grab the item from the vanilla Creative inventory instead.
4. Update Roughly Enough Items (REI) and Fabric API (or your mod loader's API) to the latest release for your Minecraft version.

Check your own log