I just got this data from my Aternos server log:

Stack Trace:
38 java.lang.IllegalArgumentException: Unable to find a field that matches {modifiers=[required: 10000, banned: 1000], type={ type input instanceof interface java.util.Map }}
39 at com.comphenix.protocol.reflect.FuzzyReflection.getField(FuzzyReflection.java:352)
40 at com.comphenix.protocol.injector.packet.PacketRegistry.createNewRegister(PacketRegistry.java:188)
41 at com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:292)
42 at com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:327)
43 at com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:120)
44 at com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
45 at com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:183)
46 at org.bukkit.craftbukkit.v1_20_R2.CraftServer.loadPlugins(CraftServer.java:442)
47 at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:219)
48 at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:954)
49 at net.minecraft.server.MinecraftServer.lambda$0(MinecraftServer.java:298)
50 at java.base/java.lang.Thread.run(Thread.java:840)
51 Dump:
52 Parameters:
53 [NULL]
54 Sender:
55 com.comphenix.protocol.ProtocolLib@4d4559e[
56 statistics=
57 packetTask=
58 tickCounter=0
59 configExpectedMod=-1
60 updater=com.comphenix.protocol.updater.SpigotUpdater@13405c4
61 redirectHandler=
62 scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@2feaa04a
63 commandProtocol=
64 commandPacket=
65 commandFilter=
66 packetLogging=
67 skipDisable=false
68 isEnabled=false
69 loader=org.bukkit.plugin.java.JavaPluginLoader@76ca7158
70 server=CraftServer{serverName=CraftBukkit,serverVersion=3950-Spigot-2888990-d485cde,minecraftVersion=1.20.2}
71 file=plugins/ProtocolLib.jar
72 description=org.bukkit.plugin.PluginDescriptionFile@71af9749
73 dataFolder=plugins/ProtocolLib
74 classLoader=org.bukkit.plugin.java.PluginClassLoader@6124a52f
75 naggable=true
76 newConfig=YamlConfiguration[path='', root='YamlConfiguration']
77 configFile=plugins/ProtocolLib/config.yml
78 logger=org.bukkit.plugin.PluginLogger@2ed87148
79 ]
80 Version:
81 ProtocolLib v5.1.0
82 Java Version:
83 17.0.9
84 Server:
85 3950-Spigot-2888990-d485cde (MC: 1.20.2)

Try a dev build: https://ci.dmulloy2.net/job/ProtocolLib

Otherwise, it may just be an incompatibility with Aternos, which we don't plan on addressing