Probably just need to make sure this.tag != null in the sync packets.
java.lang.NullPointerException: Cannot invoke "net.minecraft.nbt.CompoundTag.contains(String)" because "tag" is null at com.gildedgames.aether.core.capability.rankings.AetherRankingsCapability.deserializeSynchableNBT(AetherRankingsCapability.java:58) ~[%2380!/:?] {re:classloading} at com.gildedgames.aether.core.capability.rankings.AetherRankingsCapability.deserializeSynchableNBT(AetherRankingsCapability.java:9) ~[%2380!/:?] {re:classloading} at com.gildedgames.aether.core.network.packet.AetherRankingsSyncPacket.lambda$execute$1(AetherRankingsSyncPacket.java:38) ~[%2380!/:?] {re:classloading} at net.minecraftforge.common.util.LazyOptional.ifPresent(LazyOptional.java:150) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:mixin,re:classloading} at com.gildedgames.aether.core.network.packet.AetherRankingsSyncPacket.execute(AetherRankingsSyncPacket.java:38) ~[%2380!/:?] {re:classloading} at com.gildedgames.aether.core.network.AetherPacket$AbstractAetherPacket.lambda$handle$0(AetherPacket.java:20) ~[%2380!/:?] {re:mixin,re:classloading} at net.minecraftforge.network.NetworkEvent$Context.enqueueWork(NetworkEvent.java:208) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at com.gildedgames.aether.core.network.AetherPacket$AbstractAetherPacket.handle(AetherPacket.java:20) ~[%2380!/:?] {re:mixin,re:classloading} at net.minecraftforge.network.simple.SimpleChannel$MessageBuilder.lambda$consumer$1(SimpleChannel.java:228) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraftforge.network.simple.IndexedMessageCodec.lambda$tryDecode$3(IndexedMessageCodec.java:114) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at java.util.Optional.ifPresent(Optional.java:178) ~[?:?] {re:mixin} at net.minecraftforge.network.simple.IndexedMessageCodec.tryDecode(IndexedMessageCodec.java:114) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraftforge.network.simple.IndexedMessageCodec.consume(IndexedMessageCodec.java:148) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraftforge.network.simple.SimpleChannel.networkEventListener(SimpleChannel.java:68) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:247) ~[eventbus-5.0.7.jar%2314!/:?] {} at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:239) ~[eventbus-5.0.7.jar%2314!/:?] {} at net.minecraftforge.eventbus.EventBus.post(EventBus.java:302) ~[eventbus-5.0.7.jar%2314!/:?] {} at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283) ~[eventbus-5.0.7.jar%2314!/:?] {} at net.minecraftforge.network.NetworkInstance.dispatch(NetworkInstance.java:68) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraftforge.network.NetworkHooks.lambda$onCustomPayload$1(NetworkHooks.java:75) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at java.util.Optional.map(Optional.java:260) ~[?:?] {re:mixin} at net.minecraftforge.network.NetworkHooks.onCustomPayload(NetworkHooks.java:75) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2375%2381!/:?] {re:classloading} at net.minecraft.client.multiplayer.ClientPacketListener.handleCustomPayload(ClientPacketListener.java:1958) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A} at net.minecraft.network.protocol.game.ClientboundCustomPayloadPacket.handle(ClientboundCustomPayloadPacket.java:63) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:classloading} at net.minecraft.network.protocol.game.ClientboundCustomPayloadPacket.handle(ClientboundCustomPayloadPacket.java:7) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:classloading} at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:38) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:classloading} at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:146) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B} at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:22) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,re:computing_frames,re:classloading} at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:116) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B} at net.minecraft.util.thread.BlockableEventLoop.runAllTasks(BlockableEventLoop.java:104) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B} at net.minecraft.client.Minecraft.runTick(Minecraft.java:1039) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:aether.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A} at net.minecraft.client.Minecraft.run(Minecraft.java:668) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:aether.mixins.json:client.MinecraftMixin,pl:mixin:A,pl:runtimedistcleaner:A} at net.minecraft.client.main.Main.main(Main.java:205) ~[forge-1.18.2-40.0.32_mapped_parchment_1.18.2-2022.03.13-1.18.2-recomp.jar%2376!/:?] {re:classloading,pl:runtimedistcleaner:A} at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {} at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?] {} at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {} at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?] {} at net.minecraftforge.fml.loading.targets.ForgeClientUserdevLaunchHandler.lambda$launchService$0(ForgeClientUserdevLaunchHandler.java:38) ~[fmlloader-1.18.2-40.0.32.jar%230!/:?] {} at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.Launcher.run(Launcher.java:106) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.Launcher.main(Launcher.java:77) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23) [modlauncher-9.1.3.jar%2315!/:?] {} at cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:149) [bootstraplauncher-1.0.0.jar:?] {}