[Bug/Glitch] Server connectivity 1.7.5+ when using custom items

[Technocality]

Legacy4J Version

1.8

Mod Loader Version

No response

Bug/Glitch Occurrence Context

It's in the GC/Issues spreadsheet

Crash Report

No response

What happened wrong?

Internal Exception:io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:custom_payload'

[Technocality]

Current best theory for the cause is the world stats being messed up due to earlier world version. And deleting the stats file for the affected player seems to fix it. However, for one player who had no issues, when I attempted to join on an earlier version of MC and/or L4J (I can't remember) I was kicked due to custom payload, which was expected, however the other player was also kicked due to it, to attempt to fix it I renamed his stats file to uuid-old, which didn't work until I restarted the server. After the restart everything seemed to be working until 3-5 min later when he was randomly kicked for custom payload again, I renamed the stats to uuid-old2, and he was able to join, no restart required. This happened again and I fixed it again, then it happened for a third time, at which point I gave up renaming it. Other players on the latest L4J did not have this problem, so I suspect that it was due to him doing something in game that made his stats file have the custom payload.

During this time if someone attempted to join on an old version of Re-Console/L4J they would not be able to join and they would kick all other players on the server who had L4J installed. The players were kicked for custom payload but were able to rejoin with no problems, the outdated person was never able to join. However, if I am remembering correctly, one time it did mess up the other players and they were not able to rejoin.

Also of note, is that there was one player who was on Re-Console (0.8???, so 1.7.4 L4J) the whole time, and he did not get kicked when the others did at all and had no crashes or other disconnects.

[Technocality]

Ok, the error has been narrowed down to being caused by having a custom item in the players stats file, so in this case: "universal_shops:trade_block": 1

[JacksTheGreat]

For anybody wondering, here is a video on how to repeat this issue. It seems to kick you whenever you
A. Check leaderboard after placing the block
B. Leave and rejoin after placing the block.
C. Wait a few minutes for the stats to refresh
Right now, the current fix is to
A. Reset your player stats or remove the part where it says you have crafted the Custom block
OR
B. Disable the crafting of this block to prevent future custom payload errors