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.IllegalStateException: Unable to access field private final int net.minecraft.server.v1_8_R3.EnumProtocol.i
at com.comphenix.protocol.reflect.accessors.MethodHandleHelper.getFieldAccessor(MethodHandleHelper.java:119)
at com.comphenix.protocol.reflect.accessors.Accessors.getFieldAccessor(Accessors.java:84)
at com.comphenix.protocol.reflect.StructureModifier$$Lambda$75/000000000000000000.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:497)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:487)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:241)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
at com.comphenix.protocol.reflect.StructureModifier.getFields(StructureModifier.java:183)
at com.comphenix.protocol.reflect.StructureModifier.(StructureModifier.java:116)
at com.comphenix.protocol.reflect.StructureModifier.(StructureModifier.java:101)
at com.comphenix.protocol.injector.packet.PacketRegistry.createOldRegister(PacketRegistry.java:115)
at com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:294)
at com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:327)
at com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:120)
at com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:183)
at org.bukkit.craftbukkit.v1_8_R3.CraftServer.loadPlugins(CraftServer.java:297)
at net.minecraft.server.v1_8_R3.DedicatedServer.init(DedicatedServer.java:198)
at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:525)
at java.lang.Thread.run(Thread.java:823)
Caused by: java.lang.IllegalAccessException: illegal setter on final field
at java.lang.invoke.MethodHandles$Lookup.findSetter(MethodHandles.java:809)
at com.comphenix.protocol.reflect.accessors.MethodHandleHelper.getFieldAccessor(MethodHandleHelper.java:96)
... 24 more
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@aa060cce[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@d824444f
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@b2ab694b
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=org.bukkit.plugin.java.JavaPluginLoader@99581004
server=CraftServer{serverName=CraftBukkit,serverVersion=git-Spigot-21fe707-741a1bd,minecraftVersion=1.8.8}
file=plugins/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@44218f2a
dataFolder=plugins/ProtocolLib
classLoader=org.bukkit.plugin.java.PluginClassLoader@2fffc01c
naggable=true
ebean=
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins/ProtocolLib/config.yml
logger=org.bukkit.plugin.PluginLogger@dc795c77
]
Version:
ProtocolLib v5.1.0
Java Version:
1.8.0_272
Server:
git-Spigot-21fe707-741a1bd (MC: 1.8.8)

I already tried to change plib version to 4.7, 4.8, 5.0 and 5.1
In all of them throw the same error on startup.