Added EMI to my modpack and server, when trying to join the server i get the message:

"Connection lost
Internal exception: io.netty.handler.codec.encoderException: failed to encode packet 'clientbound/minecraft:custom_payload' (emi:ping)"

Then server spits out this error:

[22:55:24] [Netty Server IO #1/ERROR]: Error sending packet clientbound/minecraft:custom_payload io.netty.handler.codec.EncoderException: Failed to encode packet 'clientbound/minecraft:custom_payload' (emi:ping) at net.minecraft.class_9136.handler$znl000$fabric-networking-api-v1$encode(class_9136.java:547) ~[server-intermediary.jar:?] at net.minecraft.class_9136.method_56426(class_9136.java:55) ~[server-intermediary.jar:?] at net.minecraft.class_9136.encode(class_9136.java:14) ~[server-intermediary.jar:?] at net.minecraft.class_2545.method_10838(class_2545.java:26) ~[server-intermediary.jar:?] at net.minecraft.class_2545.encode(class_2545.java:12) ~[server-intermediary.jar:?] at io.netty.handler.codec.MessageToByteEncoder.write(MessageToByteEncoder.java:107) ~[netty-codec-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:881) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:863) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:968) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:856) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:113) ~[netty-codec-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:881) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:863) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:968) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:856) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.ChannelOutboundHandlerAdapter.write(ChannelOutboundHandlerAdapter.java:113) ~[netty-transport-4.1.97.Final.jar:?] at net.minecraft.class_2535$2.write(class_2535.java:530) ~[server-intermediary.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:881) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.invokeWriteAndFlush(AbstractChannelHandlerContext.java:940) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:966) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:934) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:984) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.DefaultChannelPipeline.writeAndFlush(DefaultChannelPipeline.java:1025) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.channel.AbstractChannel.writeAndFlush(AbstractChannel.java:306) ~[netty-transport-4.1.97.Final.jar:?] at net.minecraft.class_2535.method_36942(class_2535.java:350) ~[server-intermediary.jar:?] at net.minecraft.class_2535.method_52917(class_2535.java:345) ~[server-intermediary.jar:?] at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174) ~[netty-common-4.1.97.Final.jar:?] at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167) ~[netty-common-4.1.97.Final.jar:?] at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470) ~[netty-common-4.1.97.Final.jar:?] at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:569) ~[netty-transport-4.1.97.Final.jar:?] at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.97.Final.jar:?] at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.97.Final.jar:?] at java.base/java.lang.Thread.run(Thread.java:1570) [?:?] Caused by: java.lang.ClassCastException: class dev.emi.emi.network.PingS2CPacket cannot be cast to class net.minecraft.class_8711 (dev.emi.emi.network.PingS2CPacket and net.minecraft.class_8711 are in unnamed module of loader net.fabricmc.loader.impl.launch.knot.KnotClassLoader @5700d6b1) at net.minecraft.class_9139$6.encode(class_9139.java:42) ~[server-intermediary.jar:?] at net.minecraft.class_8710$1.method_56489(class_8710.java:52) ~[server-intermediary.jar:?] at net.minecraft.class_8710$1.method_56490(class_8710.java:57) ~[server-intermediary.jar:?] at net.minecraft.class_8710$1.encode(class_8710.java:39) ~[server-intermediary.jar:?] at net.minecraft.class_9139$8.encode(class_9139.java:81) ~[server-intermediary.jar:?] at net.minecraft.class_9139$9.method_56442(class_9139.java:97) ~[server-intermediary.jar:?] at net.minecraft.class_9139$9.encode(class_9139.java:87) ~[server-intermediary.jar:?] at net.minecraft.class_9136.method_56426(class_9136.java:53) ~[server-intermediary.jar:?] ... 31 more

To be fair, both rei and jei has problems also rei crashes immediately when you open inventory.
Tested with jei, works fine UNTIL you try to CTRL click icon to get cheat mode, then game crashes.

Another report of this from the discord:

From what I can tell, the payload ID isn't being registered on both ends, thus the outbound Codec is dispatching an UnknownCustomPayload codec because it can't find the codec for the not-registered Id EmiNetwork.PING.

From the fabric blog:

workaround is removing emi from the mods folder in server, works only from client thus far.

This is resolved in 1.1.9