Stack Trace:
java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @baea30a
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:357)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:199)
at java.base/java.lang.reflect.Method.setAccessible(Method.java:193)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.generateClass(StructureCompiler.java:299)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:213)
at com.comphenix.protocol.reflect.compiler.BackgroundCompiler$2.call(BackgroundCompiler.java:204)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:831)
Dump:
Parameters:
com.comphenix.protocol.reflect.StructureModifier@77ca0f80[
targetType=class net.minecraft.server.v1_16_R3.NBTTagCompound
target=
converter=
fieldType=interface java.util.Map
data=[public final java.util.Map net.minecraft.server.v1_16_R3.NBTTagCompound.map]
defaultFields={}
subtypeCache={}
customConvertHandling=false
useStructureCompiler=true
]
manager:
com.comphenix.protocol.injector.PacketFilterManager@663f2d09[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@64c75f4
packetListeners=[PacketAdapter[plugin=LibsDisguises, sending=ListeningWhitelist[priority=HIGH, packets=[ENTITY_VELOCITY[class=PacketPlayOutEntityVelocity, id=70], REL_ENTITY_MOVE[class=PacketPlayOutRelEntityMove, id=39], ENTITY_EQUIPMENT[class=PacketPlayOutEntityEquipment, id=71], NAMED_ENTITY_SPAWN[class=PacketPlayOutNamedEntitySpawn, id=4], ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=68], ATTACH_ENTITY[class=PacketPlayOutAttachEntity, id=69], ANIMATION[class=PacketPlayOutAnimation, id=5], REL_ENTITY_MOVE_LOOK[class=PacketPlayOutRelEntityMoveLook, id=40], ENTITY_LOOK[class=PacketPlayOutEntityLook, id=41], ENTITY_TELEPORT[class=PacketPlayOutEntityTeleport, id=86], ENTITY_HEAD_ROTATION[class=PacketPlayOutEntityHeadRotation, id=58], ENTITY_STATUS[class=PacketPlayOutEntityStatus, id=26], UPDATE_ATTRIBUTES[class=PacketPlayOutUpdateAttributes, id=88], ENTITY_EFFECT[class=PacketPlayOutEntityEffect, id=89]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=LibsDisguises, sending=ListeningWhitelist[priority=NORMAL, packets=[NAMED_SOUND_EFFECT[class=PacketPlayOutNamedSoundEffect, id=81], ENTITY_STATUS[class=PacketPlayOutEntityStatus, id=26]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=LibsDisguises, sending=ListeningWhitelist[priority=HIGH, packets=[WINDOW_ITEMS[class=PacketPlayOutWindowItems, id=19], HELD_ITEM_SLOT[class=PacketPlayInHeldItemSlot, id=37], SET_CREATIVE_SLOT[class=PacketPlayInSetCreativeSlot, id=40], WINDOW_CLICK[class=PacketPlayInWindowClick, id=9], SET_SLOT[class=PacketPlayOutSetSlot, id=21]], gamephase=PLAYING, options=[]], receiving=ListeningWhitelist[priority=HIGH, packets=[WINDOW_ITEMS[class=PacketPlayOutWindowItems, id=19], HELD_ITEM_SLOT[class=PacketPlayInHeldItemSlot, id=37], SET_CREATIVE_SLOT[class=PacketPlayInSetCreativeSlot, id=40], WINDOW_CLICK[class=PacketPlayInWindowClick, id=9], SET_SLOT[class=PacketPlayOutSetSlot, id=21]], gamephase=PLAYING, options=[]]]]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@215dccb2
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@4bd50b7
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@1b75624c
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@7ef34703
hasClosed=false
classLoader=PluginClassLoader{plugin=ProtocolLib v4.6.0, pluginEnabled=true, url=plugins\ProtocolLib.jar}
reporter=com.comphenix.protocol.ProtocolLib$1@5f52fa5c
server=CraftServer{serverName=Paper,serverVersion=git-Paper-652,minecraftVersion=1.16.5}
library=ProtocolLib v4.6.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@baa0d44
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=0
phasePlayingCount=4
packetCreation=false
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@4b6d5732
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@2e3a9104
hasRecycleDistance=true
minecraftVersion=(MC: 1.16.5)
debug=false
]
Sender:
com.comphenix.protocol.reflect.compiler.BackgroundCompiler@47e0bb54[
listeners={com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@c6bcc8b5=[com.comphenix.protocol.reflect.compiler.BackgroundCompiler$1@6271077d], com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@89f9ccc2=[com.comphenix.protocol.reflect.compiler.BackgroundCompiler$1@438418c6]}
listenerLock=java.lang.Object@39cc1303
compiler=com.comphenix.protocol.reflect.compiler.StructureCompiler@7b342575
enabled=false
shuttingDown=false
executor=java.util.concurrent.Executors$FinalizableDelegatedExecutorService@11948f81
reporter=com.comphenix.protocol.ProtocolLib$1@5f52fa5c
unknownPermGenBean=java.lang.Object@72b5f75d
permGenBean=
disablePermGenFraction=0.65
]
Version:
ProtocolLib v4.6.0
Java Version:
16.0.1
Server:
git-Paper-652 (MC: 1.16.5)

I was coming here to report this issue as well. I suspect many people are going to run into this since Minecraft and Paper have announced that starting with version 1.17 java 16 will be required.

This issue was resolved in build 499+.

Same here, I have also been experiencing this error. Please fix it ASAP!

@kooldamian28 See the comment directly above yours

Check @PimvanderLoos comment for fix, thanks