Stack Trace:
java.lang.IllegalArgumentException: Unable to find a field that matches {modifiers=[required: 10000, banned: 1000], type=input instanceof interface java.util.Map}
at ProtocolLib (1).jar//com.comphenix.protocol.reflect.FuzzyReflection.getField(FuzzyReflection.java:460)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.createNewRegister(PacketRegistry.java:191)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:284)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:317)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:211)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.PacketFilterBuilder.buildInternal(PacketFilterBuilder.java:185)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:177)
at ProtocolLib (1).jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:203)
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:36)
at org.bukkit.craftbukkit.v1_20_R2.CraftServer.loadPlugins(CraftServer.java:514)
at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:273)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:1086)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:315)
at java.base/java.lang.Thread.run(Thread.java:833)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@357b0bf5[
statistics=
backgroundCompiler=
packetTask=-1
tickCounter=0
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@87cde1f
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@2edddf48
redirectHandler=
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@4e0a3335
server=CraftServer{serverName=Paper,serverVersion=git-Paper-318,minecraftVersion=1.20.2}
file=plugins\ProtocolLib (1).jar
description=org.bukkit.plugin.PluginDescriptionFile@4d2113f3
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@4d2113f3
dataFolder=plugins\ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v4.8.0, pluginEnabled=false, url=plugins\ProtocolLib (1).jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins\ProtocolLib\config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@124abb96
]
Version:
ProtocolLib v4.8.0
Java Version:
17.0.8
Server:
git-Paper-318 (MC: 1.20.2)

I am having a similar problem with v5.1.0 on MC 1.20.4

[22:18:11 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:188) 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.<init>(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:36) at org.bukkit.craftbukkit.v1_20_R3.CraftServer.loadPlugins(CraftServer.java:505) at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:273) at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:1132) at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:321) at java.base/java.lang.Thread.run(Thread.java:1583) Dump: Parameters: [NULL] Sender: com.comphenix.protocol.ProtocolLib@3d448c4e[ statistics=<null> packetTask=<null> tickCounter=0 configExpectedMod=-1 updater=com.comphenix.protocol.updater.SpigotUpdater@5a4379f8 redirectHandler=<null> scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@437cb802 commandProtocol=<null> commandPacket=<null> commandFilter=<null> packetLogging=<null> skipDisable=false isEnabled=false loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@1fe4f51e server=CraftServer{serverName=Paper,serverVersion=git-Paper-381,minecraftVersion=1.20.4} file=plugins/ProtocolLib.jar description=org.bukkit.plugin.PluginDescriptionFile@3721e1b6 pluginMeta=org.bukkit.plugin.PluginDescriptionFile@3721e1b6 dataFolder=plugins/ProtocolLib classLoader=PluginClassLoader{plugin=ProtocolLib v5.1.0, pluginEnabled=false, url=plugins/ProtocolLib.jar} naggable=true newConfig=YamlConfiguration[path='', root='YamlConfiguration'] configFile=plugins/ProtocolLib/config.yml logger=com.destroystokyo.paper.utils.PaperPluginLogger@3fa68452 ] Version: ProtocolLib v5.1.0 Java Version: 21 Server: git-Paper-381 (MC: 1.20.4)

You need to use a version of ProtocolLib, which is compatible with the Minecraft Version you use.

For 1.20.4, use this version: https://ci.dmulloy2.net/job/ProtocolLib/679/artifact/build/libs/ProtocolLib.jar