[02:13:12] [Server thread/INFO]: [ProtocolLib] Loading server plugin ProtocolLib v5.2.0-SNAPSHOT-679
[02:13:12] [Server thread/WARN]: [ProtocolLib] Version (MC: 1.21.0) has not yet been tested! Proceed with caution.
[02:13:14] [Server thread/ERROR]: [ProtocolLib] Assuming package version: v1_21_R1
[02:13:14] [Server thread/ERROR]: [ProtocolLib] INTERNAL ERROR: Cannot load ProtocolLib.
If this problem hasn't already been reported, please open a ticket
at https://github.com/dmulloy2/ProtocolLib/issues with the following data:
Stack Trace:
java.lang.IllegalArgumentException: Unable to find a field that matches {modifiers=[required: 10000, banned: 1000], type={ type input instanceof interface java.util.Map }}
at ProtocolLib.jar//com.comphenix.protocol.reflect.FuzzyReflection.getField(FuzzyReflection.java:352)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.createNewRegister(PacketRegistry.java:183)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:334)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:369)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:120)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at ProtocolLib.jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:183)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:59)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:18)
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:39)
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:40)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:560)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:326)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1229)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:333)
at java.base/java.lang.Thread.run(Thread.java:1583)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@51c1da07[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@4d6e6c22
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@1220966a
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@5ab859f3
server=CraftServer{serverName=Purpur,serverVersion=1.21-2276-72ccfbd,minecraftVersion=1.21}
file=plugins.paper-remapped\ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@6f51237a
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@6f51237a
dataFolder=plugins\ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.2.0-SNAPSHOT-679, pluginEnabled=false, url=plugins.paper-remapped\ProtocolLib.jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins\ProtocolLib\config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@266bcb00
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@6d2a771c
allowsLifecycleRegistration=true
]
Version:
ProtocolLib v5.2.0-SNAPSHOT-679
Java Version:
21.0.3
Server:
1.21-2276-72ccfbd (MC: 1.21)

Should be fixed in the latest dev build so please use the latest dev build and try again: https://ci.dmulloy2.net/job/ProtocolLib/lastSuccessfulBuild/