[13:41:00] [Server thread/INFO]: [ProtocolLib] Loading server plugin ProtocolLib v5.3.0
[13:41:00] [Server thread/WARN]: [ProtocolLib] Version (MC: 1.21.5) has not yet been tested! Proceed with caution.
[13:41:00] [Server thread/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.RuntimeException: Unable to find network.ProtocolInfo$a (network.ProtocolInfo$Unbound)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.lambda$getMinecraftClass$5(MinecraftReflection.java:1581)
at java.base/java.util.Optional.orElseThrow(Optional.java:403)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.getMinecraftClass(MinecraftReflection.java:1581)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.getProtocolInfoUnboundClass(MinecraftReflection.java:1878)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.createRegisterV1_20_5(PacketRegistry.java:367)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:552)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:604)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:112)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at ProtocolLib.jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:179)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:59)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:18)
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:39)
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:39)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:567)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:235)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1243)
at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:351)
at java.base/java.lang.Thread.run(Thread.java:1575)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@300ee41f[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@3a5efb86
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.FoliaScheduler@6bbc9f6c
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@d1e48bd
server=CraftServer{serverName=Folia,serverVersion=1.21.5-DEV-dfa3ca4,minecraftVersion=1.21.5}
file=plugins/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@59f6cdaf
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@59f6cdaf
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.3.0, pluginEnabled=false, url=plugins/ProtocolLib.jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins/ProtocolLib/config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@40653e4b
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@2f91185d
allowsLifecycleRegistration=true
isBeingEnabled=false
]
Version:
ProtocolLib v5.3.0
Java Version:
23.0.2
Server:
1.21.5-DEV-dfa3ca4 (MC: 1.21.5)

I'm getting this too.

[01:08:11 INFO]: [ProtocolLib] Loading server plugin ProtocolLib v5.3.0
[01:08:11 WARN]: [ProtocolLib] Version (MC: 1.21.5) has not yet been tested! Proceed with caution.
[01:08:11 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.RuntimeException: Unable to find network.ProtocolInfo$a (network.ProtocolInfo$Unbound)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.lambda$getMinecraftClass$5(MinecraftReflection.java:1581)
at java.base/java.util.Optional.orElseThrow(Optional.java:403)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.getMinecraftClass(MinecraftReflection.java:1581)
at ProtocolLib.jar//com.comphenix.protocol.utility.MinecraftReflection.getProtocolInfoUnboundClass(MinecraftReflection.java:1878)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.createRegisterV1_20_5(PacketRegistry.java:367)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:552)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:604)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:112)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at ProtocolLib.jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:179)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:59)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:18)
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:39)
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:39)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:567)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:235)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1161)
at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:308)
at java.base/java.lang.Thread.run(Thread.java:1583)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@4e6d31bb[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@5097319b
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@7b85e8a9
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@5c310cc2
server=CraftServer{serverName=Paper,serverVersion=1.21.5-113-ba7fb23,minecraftVersion=1.21.5}
file=plugins/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@2f2d777b
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@2f2d777b
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.3.0, pluginEnabled=false, url=plugins/ProtocolLib.jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins/ProtocolLib/config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@41e968d0
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@5bb4939
allowsLifecycleRegistration=true
isBeingEnabled=false
]
Version:
ProtocolLib v5.3.0
Java Version:
21.0.7
Server:
1.21.5-113-ba7fb23 (MC: 1.21.5)