1. 1.20.1, 1.2.6 rc9
2. Mutiblock machines (fission, fusion) are not forming on a server, but they worked in a single-player world.

i can't confirm this. tested on 2 servers with NCN rc9
i will need more details

I have a huge mod list, and there are some weird mods that might have done something weird. One suspect is ModernFix, but I was not able to verify it.

Here is the server debug log:

debug.log

Please try this experiment:
in config/NuclearCraft//common.toml
set these values:

[Misc]
#Debug logging. Enable in case of issues to collect more data
debug_logging = true
#Speeds up blocks indexing of multiblocks
#Gives big performance boost for large multiblocks
#Disable in case of issues
experimental_block_indexing = false

So then server log will have much more info + it will try to use basic multiblock validation algo
@Tonywww2

Here it is:

debug.log

are you running on java 19?

Yes, OpenJDK 19

Don't think this will be a problem. I guess this is because world is lagging. And NC handler has check if world tick has time.
I removed that check
checkout this build on your server
https://github.com/igentuman/NuclearCraft-Neoteric/actions/runs/17489547225

This build works, thank you!