This came up on my survival world this morning:

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 @b9fa1e
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@7d7b4ef6[
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@22ba8556[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@1ec10bbd
packetListeners=[PacketAdapter[plugin=SuperVanish, sending=ListeningWhitelist[priority=HIGH, packets=[PLAYER_INFO[class=PacketPlayOutPlayerInfo, id=50]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=SuperVanish, sending=ListeningWhitelist[priority=NORMAL, packets=[OUT_SERVER_INFO[class=PacketStatusOutServerInfo, id=0]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=SuperVanish, sending=ListeningWhitelist[priority=NORMAL, packets=[PLAYER_INFO[class=PacketPlayOutPlayerInfo, id=50]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=SuperVanish, sending=ListeningWhitelist[priority=LOW, packets=[ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=68], ABILITIES[class=PacketPlayOutAbilities, id=48], PLAYER_INFO[class=PacketPlayOutPlayerInfo, id=50]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST]]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@396aea6d
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@68dcf22e
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@1eba4a56
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@8b036df
hasClosed=false
classLoader=PluginClassLoader{plugin=ProtocolLib v4.6.0, pluginEnabled=true, url=plugins\ProtocolLib.jar}
reporter=com.comphenix.protocol.ProtocolLib$1@66e79c26
server=CraftServer{serverName=Tuinity,serverVersion=git-Tuinity-"f7c4249",minecraftVersion=1.16.5}
library=ProtocolLib v4.6.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@258140ce
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=1
phasePlayingCount=5
packetCreation=true
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@1ae6ac40
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@4253b687
hasRecycleDistance=true
minecraftVersion=(MC: 1.16.5)
debug=false
]
Sender:
com.comphenix.protocol.reflect.compiler.BackgroundCompiler@257d743a[
listeners={com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@ceef2ad=[com.comphenix.protocol.reflect.compiler.BackgroundCompiler$1@609e7327]}
listenerLock=java.lang.Object@4f3557d8
compiler=com.comphenix.protocol.reflect.compiler.StructureCompiler@100d3737
enabled=false
shuttingDown=false
executor=java.util.concurrent.Executors$FinalizableDelegatedExecutorService@5e1f5da8
reporter=com.comphenix.protocol.ProtocolLib$1@66e79c26
unknownPermGenBean=java.lang.Object@4d8b1a07
permGenBean=
disablePermGenFraction=0.65
]
Version:
ProtocolLib v4.6.0
Java Version:
16.0.1
Server:
git-Tuinity-"f7c4249" (MC: 1.16.5)

No Idea what it means but it said to report it here.

#1190

https://ci.dmulloy2.net/job/ProtocolLib/505/
This version is needed for useage with Java 16 - its 4.6.1 build 505