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:168)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:292)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:327)
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:546)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:292)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1215)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:330)
at java.base/java.lang.Thread.run(Thread.java:1583)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@415f1aa[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@75929d06
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@5e21babf
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@33085b84
server=CraftServer{serverName=Paper,serverVersion=1.21.1-74-971a7a5,minecraftVersion=1.21.1}
file=plugins/.paper-remapped/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@c5859fa
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@c5859fa
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.1.0, 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@345bb5a8
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@5b32d09a
allowsLifecycleRegistration=true
]
Version:
ProtocolLib v5.1.0
Java Version:
21.0.4
Server:
1.21.1-74-971a7a5 (MC: 1.21.1)

Should be fixed in the latest release so please use the latest release and try again: https://github.com/dmulloy2/ProtocolLib/releases/latest