My friend has been hosting a modded server, containing a lot of Team Abnormals mods on 1.21, and multiple users have encountered an issue when joining the server will kick them out over an issue with the minecraft:clientbound/custom_payload packet, seemingly out of nowhere to those who experience it.

Originally I believed this was an issue related to Blueprint, and reported the issue here (team-abnormals/blueprint#308 (comment)), but was told it could be a version mismatch (this isn't the case as everyone's on the same version) or it could be one of the mods dependent on it.

I later discovered an issue with Boadload with this same packet on the discord that was fixed recently. (https://discord.com/channels/650003402218274816/729290806493708290/1366793285804490863)

Besides the report found in the blueprint issue (https://pastebin.com/paM5iND9), it seems there's also another variant where instead of complaining about the length of the packet, it's about 3 missing bytes. Could it be that the issue fixed in 6.0.1 still persists on dedicated servers? (https://pastebin.com/KAwMmGpD)

^ this