Here is the disconnect text
disconnect-2025-01-19_14.08.22-client.txt

Created a new Fabulously Optimized instance on 1.21.4 with Subtle Effects and a new local server with the Fabulously Optimized mods. I tried different combinations of restarting the server and client and adding/removing Subtle Effect and the Fabulously Optimized pack. I even messed around with the composter, since the log still says that's part of the issue. I had no issues.

RubenRME says that updating mods fixed it, and I was using the latest versions of everything, so try updating your mods to see if that works

I am using the latest version of every mod on my client and server. For some added context, the block that the error message refers to (-386,64,8) is surrounded by hoppers being used for a moss-based bonemeal farm. I inspected the region file in NBTExplorer and I don't see any unusual data in either the block_entities or entries in block_states.palette. It's possible the server is sending some data outside of my world file, but I can't imagine what would be the cause. I'll continue troubleshooting and share if I find anything else out.

There's definitely something on the server that is causing this issue to occur. I made a single player copy of this world, and was able to load and visit the offending block. It's exactly at this composter between two hoppers that the error message indicates is a void_air block if I'm understanding that correctly. I don't even think Fabulously Optimized has much to do with the server side outside of maybe Lithium. I guess the next thing to check would be loading this region data on a vanilla server.

It turns out I did add Krypton as a mod which optimizes the networking stack, and that removing it causes the issue to go away. Given the nature of this mod, I'm going to assume the issue lies with them.

It works fine for me. Is this a vanilla or modded server? are there any plugins or mods that change the composter block (thats what the disconnect log says is the issue) in some way?

In my case the server is running the same mod loadout as Fabulously Optimized, which should not change anything to do with Composters as far as I'm aware.

Weirdly enough this is now fixed for me. I have since updated the following mods on the server side: lithium, crops-love-rain, betterstats, and fabric-api. I tried reverting lithium and the fabric api to their respective versions from before, but I am genuinely unable to replicate the issue...

I hadn't tested this since initially reporting it, but it also looks like the issue is resolved for me.

Oops, never mind. I closed too soon.
disconnect-2025-01-19_14.02.09-client.txt

Same error here, not sure what changed cause I was able to join on 1.8.0 before.