I have the exact same jar "geckolib-forge-1.20.1-4.2.1.jar" on both client and server, but the server rejects connections due to this mod not having the correct version.

The only mention of the mod in the server log is

[19Jul2023 14:57:21.918] [main/DEBUG] [net.minecraftforge.fml.loading.moddiscovery.ModFileParser/LOADING]: Considering mod file candidate /home/container/mods/geckolib-forge-1.20.1-4.2.1.jar
[19Jul2023 14:57:21.918] [main/DEBUG] [net.minecraftforge.fml.loading.moddiscovery.ModFileInfo/LOADING]: Found valid mod file geckolib-forge-1.20.1-4.2.1.jar with {geckolib} mods - versions {4.2.1}

[19Jul2023 14:57:22.149] [main/DEBUG] [net.minecraftforge.fml.loading.moddiscovery.AbstractJarFileDependencyLocator/]: Failed to load resource META-INF/jarjar/metadata.json from geckolib-forge-1.20.1-4.2.1.jar, it does not contain dependency information.

 [19Jul2023 14:57:22.282] [main/WARN] [net.minecraftforge.jarjar.selection.JarSelector/]: Attempted to select a dependency jar for JarJar which was passed in as source: geckolib. Using Mod File: /home/container/mods/geckolib-forge-1.20.1-4.2.1.jar

Forge 47.1.3

Provide your full client and server log files

I apologize, but after deleting the modpack in the client and reinstalling, and then loading the backup of the world prior to updating the server (and deleting config) I am not able to reproduce. While geckolib was deleted in the pack's mods/ folder it somehow still showed up in the loaded modlist in-game. I can't begin to guess as to how. Unless modlist is cached somehow... Logs are attached from when it was occurring, but I'm guessing this should be closed.

debug.log
latest.log

Yeah with no reproduction we can't say this is anything more than a random conflict or glitch with caching or something

We'll see what happens I guess

Apparently it does eventually come back... Modpack version and server haven't changed. Once it starts then all users experience the same thing. I did notice an apparently vanilla connection attempt, but not sure that's related.

server - debug.log
client-debug.log

pack mod list
server mod list

I'll do anything I can to dig further.

Finally figured something useful out.
Ars Nouveau is also a dependent on Geckolib, even though CurseForge does not list that... It had a recent update with "Geckolib is now JIJ instead of shaded" in the changelog. I don't know what that means, but with Ars 4.2.0-all the Geckolib version is rejected by the server even when both server and client have the same geckolib jar as I mentioned before, showing up as this in the client debug log:

[Netty Client IO #2/DEBUG] [net.minecraftforge.network.NetworkRegistry/NETREGISTRY]: Channel 'geckolib:main' : Version test of 'ABSENT ?' from server : REJECTED

This does not occur, and my issue seems to be fixed 🤞, when downgrading Ars to 4.1.1. Can a mod load its dependencies without the client installing them? I am completely unknowledgeable of such things. But Occultism also depends on Geckolib so the separate geckolib jar does need to be there... Anyway thanks. @Tslat