• This issue is not solved in a development build

Describe the bug
Error on server startup

To Reproduce
Steps to reproduce the behavior:

1. Run server
2. See error

Expected behavior
Load ProtocolLib on server startup

Screenshots
https://i.imgur.com/IeRGxzY.png

[10:13:42 WARN]: [ProtocolLib] Version (MC: 1.20.4) has not yet been tested! Proceed with caution.
[10:13:44 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:519)
at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:307)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:1146)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:324)
at java.base/java.lang.Thread.run(Thread.java:833)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@3f178710[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@7b49ba15
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@2640a26b
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@280491b0
server=CraftServer{serverName=Purpur,serverVersion=git-Purpur-2107,minecraftVersion=1.20.4}
file=plugins\ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@1d8732c5
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@1d8732c5
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@32f2178b
]
Version:
ProtocolLib v5.1.0
Java Version:
17.0.8
Server:
git-Purpur-2107 (MC: 1.20.4)

I had the same issue on same java version and Minecraft version as you. My solution was to simply use the version downloaded from here: https://ci.dmulloy2.net/job/ProtocolLib//lastBuild/
I don't believe the link was properly updated on the Spigot page.