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 @9ffa361
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@6febd9d2[
targetType=class net.minecraft.server.v1_16_R3.PacketPlayOutChat
target=
converter=
fieldType=class java.lang.Object
data=[private net.minecraft.server.v1_16_R3.IChatBaseComponent net.minecraft.server.v1_16_R3.PacketPlayOutChat.a, public net.kyori.adventure.text.Component net.minecraft.server.v1_16_R3.PacketPlayOutChat.adventure$message, public net.md_5.bungee.api.chat.BaseComponent[] net.minecraft.server.v1_16_R3.PacketPlayOutChat.components, private net.minecraft.server.v1_16_R3.ChatMessageType net.minecraft.server.v1_16_R3.PacketPlayOutChat.b, private java.util.UUID net.minecraft.server.v1_16_R3.PacketPlayOutChat.c]
defaultFields={public net.md_5.bungee.api.chat.BaseComponent[] net.minecraft.server.v1_16_R3.PacketPlayOutChat.components=2, private net.minecraft.server.v1_16_R3.ChatMessageType net.minecraft.server.v1_16_R3.PacketPlayOutChat.b=3, private java.util.UUID net.minecraft.server.v1_16_R3.PacketPlayOutChat.c=4}
subtypeCache={class java.util.UUID=StructureModifier[fieldType=class java.util.UUID, data=[private java.util.UUID net.minecraft.server.v1_16_R3.PacketPlayOutChat.c]]}
customConvertHandling=false
useStructureCompiler=true
]
manager:
com.comphenix.protocol.injector.PacketFilterManager@ec2bc4e[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@4b6144e5
packetListeners=[PacketAdapter[plugin=HolographicDisplays, sending=ListeningWhitelist[priority=NORMAL, packets=[SPAWN_ENTITY[class=PacketPlayOutSpawnEntity, id=0], SPAWN_ENTITY_LIVING[class=PacketPlayOutSpawnEntityLiving, id=2], ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=68], REL_ENTITY_MOVE[class=PacketPlayOutRelEntityMove, id=39], REL_ENTITY_MOVE_LOOK[class=PacketPlayOutRelEntityMoveLook, id=40]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=AdvancedEnchantments, sending=ListeningWhitelist[priority=HIGHEST, packets=[SPAWN_ENTITY[class=PacketPlayOutSpawnEntity, id=0], SPAWN_ENTITY_LIVING[class=PacketPlayOutSpawnEntityLiving, id=2]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], com.bergerkiller.bukkit.common.internal.network.ProtocolLibPacketHandler$CommonPacketListener@2c6b4f43, PacketAdapter[plugin=AdvancedEnchantments, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=HIGHEST, packets=[USE_ENTITY[class=PacketPlayInUseEntity, id=14]], gamephase=PLAYING, options=[]]], com.bergerkiller.bukkit.common.internal.network.ProtocolLibPacketHandler$CommonPacketListener@7b1c3210, com.bergerkiller.bukkit.common.internal.network.ProtocolLibPacketHandler$CommonPacketMonitor@5042405c]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@7af53112
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@f0c240e
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@1437c96d
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@55b0e162
hasClosed=false
classLoader=PluginClassLoader{plugin=ProtocolLib v4.6.0, pluginEnabled=true, url=plugins/ProtocolLib.jar}
reporter=com.comphenix.protocol.ProtocolLib$1@61e2767
server=CraftServer{serverName=Paper,serverVersion=git-Paper-788,minecraftVersion=1.16.5}
library=ProtocolLib v4.6.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@428a8f39
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=0
phasePlayingCount=8
packetCreation=false
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@7bfd5ce5
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@75ffda64
hasRecycleDistance=true
minecraftVersion=(MC: 1.16.5)
debug=false
]
Sender:
com.comphenix.protocol.reflect.compiler.BackgroundCompiler@1d0a54d9[
listeners={com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@48dc343e=[com.comphenix.protocol.reflect.compiler.BackgroundCompiler$1@3711d3e5], com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@25e59b69=[com.comphenix.protocol.injector.StructureCache$1@269abb96]}
listenerLock=java.lang.Object@2dbd79db
compiler=com.comphenix.protocol.reflect.compiler.StructureCompiler@625e5c29
enabled=false
shuttingDown=false
executor=java.util.concurrent.Executors$FinalizableDelegatedExecutorService@5b69357c
reporter=com.comphenix.protocol.ProtocolLib$1@61e2767
unknownPermGenBean=java.lang.Object@285c738c
permGenBean=
disablePermGenFraction=0.65
]
Version:
ProtocolLib v4.6.0
Java Version:
16.0.2
Server:
git-Paper-788 (MC: 1.16.5)

This is fixed in the 4.7.0 release. Find it here: https://github.com/dmulloy2/ProtocolLib/releases/tag/4.7.0