java.lang.IllegalArgumentException: Unable to find NetworkManager in [id: 0x8e39b88b, L:/XX.XX.XX.XX:25565 - R:/XX.XX.XX.XX:46196]
at com.comphenix.protocol.injector.netty.InjectionFactory.findNetworkManager(InjectionFactory.java:209)
at com.comphenix.protocol.injector.netty.InjectionFactory.fromChannel(InjectionFactory.java:142)
at com.comphenix.protocol.injector.netty.ProtocolInjector$1.initChannel(ProtocolInjector.java:157)
at io.netty.channel.ChannelInitializer.initChannel(ChannelInitializer.java:129)
at io.netty.channel.ChannelInitializer.handlerAdded(ChannelInitializer.java:112)
at io.netty.channel.AbstractChannelHandlerContext.callHandlerAdded(AbstractChannelHandlerContext.java:1114)
at io.netty.channel.DefaultChannelPipeline.callHandlerAdded0(DefaultChannelPipeline.java:609)
at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:223)
at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:381)
at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:370)
at com.comphenix.protocol.injector.netty.ProtocolInjector$2.initChannel(ProtocolInjector.java:171)
at io.netty.channel.ChannelInitializer.initChannel(ChannelInitializer.java:129)
at io.netty.channel.ChannelInitializer.handlerAdded(ChannelInitializer.java:112)
at io.netty.channel.AbstractChannelHandlerContext.callHandlerAdded(AbstractChannelHandlerContext.java:1114)
at io.netty.channel.DefaultChannelPipeline.callHandlerAdded0(DefaultChannelPipeline.java:609)
at io.netty.channel.DefaultChannelPipeline.access$100(DefaultChannelPipeline.java:46)
at io.netty.channel.DefaultChannelPipeline$PendingHandlerAddedTask.execute(DefaultChannelPipeline.java:1463)
at io.netty.channel.DefaultChannelPipeline.callHandlerAddedForAllHandlers(DefaultChannelPipeline.java:1115)
at io.netty.channel.DefaultChannelPipeline.invokeHandlerAddedIfNeeded(DefaultChannelPipeline.java:650)
at io.netty.channel.AbstractChannel$AbstractUnsafe.register0(AbstractChannel.java:514)
at io.netty.channel.AbstractChannel$AbstractUnsafe.access$200(AbstractChannel.java:429)
at io.netty.channel.AbstractChannel$AbstractUnsafe$1.run(AbstractChannel.java:486)
at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470)
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:406)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.lang.Thread.run(Thread.java:750)
Dump:
manager:
com.comphenix.protocol.injector.PacketFilterManager@565c282[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@302dfdfa
packetListeners=[]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@5d0731ae
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@5d874663
interceptWritePacket=com.comphenix.protocol.injector.packet.InterceptWritePacket@765dee83
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@29401f9f
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@4daed158
hasClosed=false
classLoader=org.bukkit.plugin.java.PluginClassLoader@1e1177d3
reporter=com.comphenix.protocol.ProtocolLib$1@2a7d32d0
server=CraftServer{serverName=Verasity,serverVersion=1.8.8-R0.1-SNAPSHOT,minecraftVersion=1.8.8}
library=ProtocolLib v4.5.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@1ad8b5fb
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=0
phasePlayingCount=0
packetCreation=false
spigotInjector=
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@61117dfd
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@7cd462fd
hasRecycleDistance=true
minecraftVersion=(MC: 1.8.8)
loginPackets=com.comphenix.protocol.injector.LoginPackets@60523192
debug=false
]
Sender:
com.comphenix.protocol.injector.netty.ProtocolInjector@61117dfd[
injected=true
closed=false
playerFactory=com.comphenix.protocol.injector.server.TemporaryPlayerFactory@151b81b4
bootstrapFields=[VolatileField [accessor=com.comphenix.protocol.reflect.accessors.Accessors$SynchronizedFieldAccessor@2013bee9, container=net.minecraft.server.v1_8_R3.ServerConnection@51bcd5d8, previous=io.github.repooper.packetevents.Vulcan_Tz@18e3f87a, current=com.comphenix.protocol.injector.netty.BootstrapList@182f2719, previousLoaded=true, currentSet=true, forceAccess=false]]
injectionFactory=com.comphenix.protocol.injector.netty.InjectionFactory@25f62603
networkManagers=io.github.repooper.packetevents.Vulcan_Tz@12d33570
sendingFilters=com.comphenix.protocol.concurrency.PacketTypeSet@15742f78
reveivedFilters=com.comphenix.protocol.concurrency.PacketTypeSet@7482d520
mainThreadFilters=com.comphenix.protocol.concurrency.PacketTypeSet@6050338d
bufferedPackets=com.comphenix.protocol.concurrency.PacketTypeSet@6b23e242
invoker=com.comphenix.protocol.injector.PacketFilterManager@565c282
reporter=com.comphenix.protocol.ProtocolLib$1@2a7d32d0
debug=false
]
Version:
ProtocolLib v4.5.0
Java Version:
1.8.0_352
Server:
1.8.8-R0.1-SNAPSHOT (MC: 1.8.8)

Please try if this problem persists in the latest release (https://www.spigotmc.org/resources/protocollib.1997/download?version=443701)