[09:29:40 WARN]: [ProtocolLib] Version (MC: 1.20.4) has not yet been tested! Proceed with caution.
[09:29:40 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.(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:509)
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:840)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@722387ec[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@2427038
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@28f4e093
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@336ac03
server=CraftServer{serverName=Paper,serverVersion=git-Paper-329,minecraftVersion=1.20.4}
file=plugins/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@7c96395e
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@7c96395e
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@450952fd
]
Version:
ProtocolLib v5.1.0
Java Version:
17.0.9
Server:
git-Paper-329 (MC: 1.20.4)

[09:29:46 INFO]: [ProtocolLib] Enabling ProtocolLib v5.1.0
[09:29:46 ERROR]: ******************************************************
[09:29:46 ERROR]: *** ProtocolLib does not support plugin reloaders! ***
[09:29:46 ERROR]: *** Please use the built-in reload command! ***
[09:29:46 ERROR]: ******************************************************
[09:29:46 INFO]: [ProtocolLib] Disabling ProtocolLib v5.1.0

I also have such problem with paper 1.20.4

I get same error as you but i don't even use PlugMan etc. something.
Version: ProtocolLib v5.1.0 Java Version: 17.0.8 Server: git-Leaves-"10acfe9" (MC: 1.20.2)

As stated in the message, you are using an old version of ProtocolLib that is not compatible with 1.20.4.

Use this development build instead: https://ci.dmulloy2.net/job/ProtocolLib/679/artifact/build/libs/ProtocolLib.jar