This error happens. For ThreepennyLlama he loses connection. For me it crashes the server. Error message is the same though.
[12Sep2021 17:11:46.618] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]:
ThreepennyLlama joined the game
[12Sep2021 17:11:46.997] [Netty Epoll Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]:
Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@6b6b2a2c (class dev.maxneedssnacks.interactio.command.RegistryArgument) -
will not be sent to client!
[12Sep2021 17:11:46.999] [Server thread/INFO] [se.mickelus.tetra.data.DataManager/]:
Sending data to client: ThreepennyLlama
[12Sep2021 17:11:47.085] [Netty Epoll Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]:
Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@6b6b2a2c (class dev.maxneedssnacks.interactio.command.RegistryArgument) -
will not be sent to client!
This error happens. For ThreepennyLlama he loses connection. For me it crashes the server. Error message is the same though. [12Sep2021 17:11:46.618] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: ThreepennyLlama joined the game
[12Sep2021 17:11:46.997] [Netty Epoll Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@6b6b2a2c (class dev.maxneedssnacks.interactio.command.RegistryArgument) - will not be sent to client!
[12Sep2021 17:11:46.999] [Server thread/INFO] [se.mickelus.tetra.data.DataManager/]: Sending data to client: ThreepennyLlama
[12Sep2021 17:11:47.085] [Netty Epoll Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@6b6b2a2c (class dev.maxneedssnacks.interactio.command.RegistryArgument) - will not be sent to client!
[12Sep2021 17:11:47.227] [Server thread/ERROR] [com.lothrazar.cyclic.ModCyclic/]: ? overwrite PlayerEvent.LoadFromFile {inv:{Size:36,Items:[]},spectatorTicks:0,flyTicks:0,storage:0b,tasks:[]}
[12Sep2021 17:11:47.227] [Server thread/ERROR] [com.lothrazar.cyclic.ModCyclic/]: C PlayerEvent.LoadFromFile {inv:{Size:36,Items:[]},spectatorTicks:0,flyTicks:0,storage:0b,tasks:[]}
[12Sep2021 17:12:10.283] [Server thread/INFO] [net.minecraft.network.play.ServerPlayNetHandler/]: ThreepennyLlama lost connection: Timed out
[12Sep2021 17:12:10.283] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: ThreepennyLlama left the game