I have tried that...

• I am using the latest Arclight build

• I have read the FAQ and Troubleshooting, they didn't solve my issue

• I have searched existing Issues and found no duplicates

• The mods and/or plugins I use support my Minecraft version

• I have installed all required dependencies for the mods and/or plugins

• I have updated all mods and/or plugins to their latest versions

• This issue cannot be reproduced on a Spigot server
  (Mod-related issues are considered non-reproducible on Spigot)

• This issue cannot be reproduced on the Mod Loader server (Forge/NeoForge/Fabric)
  (Plugin-related issues are considered non-reproducible on mod loaders)

Minecraft Version

1.21.1

Arclight Version

arclight-fabric-1.21.1-1.0.2-SNAPSHOT-4f2d372.jar

OS Version

Debian

Java Version

openjdk 21.0.10 2026-01-20 LTS
OpenJDK Runtime Environment Temurin-21.0.10+7 (build 21.0.10+7-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.10+7 (build 21.0.10+7-LTS, mixed mode, sharing)

Related Mods and/or Plugins (Name & Version)

architectury-13.0.8-fabric.jar
ChunkyBorder-1.2.18.jar
Chunky-Fabric-1.4.23.jar
Cobblemon-fabric-1.7.3+1.21.1.jar
fabric-api-0.116.10+1.21.1.jar
ferritecore-7.0.3-fabric.jar
lithium-fabric-0.15.3+mc1.21.1.jar
spark-1.10.109-fabric.jar
squaremap-fabric-mc1.21.1-1.3.2.jar
wondertrade-fabric-1.6.0.jar

Issue Description

Occasionally, something will happen that prevents players from joining the server. Players already on the server can continue playing but cannot rejoin if they leave. Restarting the server fixes this until it breaks again.

Server threadEx[06Apr2026 11:32:53.855] [Server thread/INFO] [net.minecraft.class_8610/]: com.mojang.authlib.GameProfile@e535d08[id=57d4eff3-67ea-40ab-8fe3-9488b77f3001,name=Immortal86524,properties={textures=[Property[name=textures, value=ewogICJ0aW1lc3RhbXAiIDogMTc3NTQ3NTE3MzA1NywKICAicHJvZmlsZUlkIiA6ICI1N2Q0ZWZmMzY3ZWE0MGFiOGZlMzk0ODhiNzdmMzAwMSIsCiAgInByb2ZpbGVOYW1lIiA6ICJJbW1vcnRhbDg2NTI0IiwKICAic2lnbmF0dXJlUmVxdWlyZWQiIDogdHJ1ZSwKICAidGV4dHVyZXMiIDogewogICAgIlNLSU4iIDogewogICAgICAidXJsIiA6ICJodHRwOi8vdGV4dHVyZXMubWluZWNyYWZ0Lm5ldC90ZXh0dXJlL2VhN2NmNDE3YjhmN2M5NzY2MmJkMDUwYWM3NWUyYmQ3MTY2ZjUwNWFhODc2MGE0YjgxNjdlNzA1N2Q4YjQwMDEiCiAgICB9LAogICAgIkNBUEUiIDogewogICAgICAidXJsIiA6ICJodHRwOi8vdGV4dHVyZXMubWluZWNyYWZ0Lm5ldC90ZXh0dXJlLzI4ZGU0YTgxNjg4YWQxOGI0OWU3MzVhMjczZTA4NmMxOGYxZTM5NjY5NTYxMjNjY2I1NzQwMzRjMDZmNWQzMzYiCiAgICB9CiAgfQp9, signature=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]]}] lost connection: Internal Exception: java.lang.NullPointerException: Cannot invoke "net.minecraft.class_2960.toString()" because the return value of "it.unimi.dsi.fastutil.objects.Object2IntMap$Entry.getKey()" is null