Stack Trace:
java.lang.RuntimeException: Unable to find network.ProtocolInfo$a (network.ProtocolInfo$Unbound)
at ProtocolLib (1).jar//com.comphenix.protocol.utility.MinecraftReflection.lambda$getMinecraftClass$5(MinecraftReflection.java:1581)
at java.base/java.util.Optional.orElseThrow(Optional.java:403)
at ProtocolLib (1).jar//com.comphenix.protocol.utility.MinecraftReflection.getMinecraftClass(MinecraftReflection.java:1581)
at ProtocolLib (1).jar//com.comphenix.protocol.utility.MinecraftReflection.getProtocolInfoUnboundClass(MinecraftReflection.java:1878)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.createRegisterV1_20_5(PacketRegistry.java:367)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:552)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:604)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:112)
at ProtocolLib (1).jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at ProtocolLib (1).jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:179)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:59)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:18)
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:39)
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:39)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:581)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:269)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1179)
at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:313)
at java.base/java.lang.Thread.run(Thread.java:1583)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@70119aa4[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@6cfc1a8c
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@1d96d828
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@4b9cf2eb
server=CraftServer{serverName=Purpur,serverVersion=1.21.8-2484-520a7e5,minecraftVersion=1.21.8}
file=plugins/ProtocolLib (1).jar
description=org.bukkit.plugin.PluginDescriptionFile@bd025b
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@bd025b
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.3.0, 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@6af5f88c
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@676418b7
allowsLifecycleRegistration=true
isBeingEnabled=false
]
Version:
ProtocolLib v5.3.0
Java Version:
21.0.6
Server:
1.21.8-2484-520a7e5 (MC: 1.21.8)

Experiencing the same issue on this version;

Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@10b8fa63[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@6f223026
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@c3ee97a
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@321db2a2
server=CraftServer{serverName=Paper,serverVersion=1.21.8-3-6fb36e3,minecraftVersion=1.21.8}
file=plugins/ProtocolLib (4).jar
description=org.bukkit.plugin.PluginDescriptionFile@7707570c
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@7707570c
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.3.0, pluginEnabled=false, url=plugins/ProtocolLib (4).jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins/ProtocolLib/config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@3bfcd0e1
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@760e4fa5
allowsLifecycleRegistration=true
isBeingEnabled=false
]
Version:
ProtocolLib v5.3.0
Java Version:
21.0.7
Server:
1.21.8-3-6fb36e3 (MC: 1.21.8)

https://github.com/dmulloy2/ProtocolLib/releases/tag/5.4.0