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• This is a crash. Please upload, Pastebin, Gist or copypaste the whole crash report along with this issue.

• This is a mod incompatibility. If I do this in vanilla Forge with only Et Futurum Requiem installed, it works normally.

Version number of Et-Futurum-Requiem (IMPORTANT)

Et_Futurum_Requiem-2.5.1+nomixin.jar
Version: 2.5.1

Describe the issue (IMPORTANT)

Client Loads up like normal going through initialising mods but always hits the error:
Error: cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/network/play/server/S1EPacketRemoveEntityEffect. I assumed that this was a library or dependency error but came out unsuccessful, I only get this error when using Et Futurum Requiem, removing the mod completely will let the client load like normal.

Mod list (OPTIONAL)

Crash Report: https://toolbox.vee.city/opengist/t2vee/20be127fc7064637a6021f1052708da4

Forge Client Log: https://toolbox.vee.city/opengist/t2vee/32abec16ce2e4d7a81f0f3428d04e850

Additional Context (OPTIONAL)

I am using +unimixins-all-1.7.10-0.1.14.jar with the +nomixin version but have also tried the version with and receive the same error.

Are you sure EFR is related to this? It seems to me that the issue is that you're using two incompatible potion extension mods, EndlessIDs and ExtendedPotions.

[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]: Found S1EPacketRemoveEntityEffect Class: net/minecraft/network/play/server/S1EPacketRemoveEntityEffect
[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]:  - Changed getId to Integer
[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]:  - Patching readPacketData
[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]:  - Changed reading to readInt
[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]:  - Patching writePacketData
[02:10:29] [Client thread/DEBUG] [ExtendedPotionsCore/etfuturum]:  - Changed writing to writeInt
[02:10:29] [Client thread/DEBUG] [mixin/etfuturum]: Mixing common.potion.vanilla.network.S1EPacketRemoveEntityEffectMixin from mixins.endlessids.json into net.minecraft.network.play.server.S1EPacketRemoveEntityEffect

EFR probably just happens to be the first mod that tries to access the class that failed to get transformed, and the same thing would happen without it if you tried to use some potions in-game.

I am working on a project that uses EFR as a base. When adding other mods to the project I encountered a similar error. This could be caused. by conflicting mixins. If you're using a mixin bootstrap mod, (which obviously you have to be if you're using the nomixin build) make sure that no other mods you have installed shade mixin and download coretweaks and try running again. Coretweaks will most likely show you the real error. 90% of the time these missing class or method errorsvaren't real errors. Do note I did not read the whole thing I'm at work right now. Makamys said something about potion ids, that could very well be the issue too. Hope this helps.

CoreTweaks's fix is unlikely to be relevant here, there's no log4j in the stacktrace.

As well as following makamys' suggestion, what happens if you remove te reset fix? nvm, the packet in question is to remove a potion effect from an entity on the client (used by levitation since that needs to be synced to the client)

Are you able to narrow down possible culprit mods?

Ah, following @makamys solution, it appears that ExtendedPotions was causing the crashes. Removing it lets the client launch like normal.
Apologies for opening the issue for a unrelated problem, I would of never of found that so thank you anyways!