I have a problem with the luckyblock mod, I created a server in 1.16.5 with forge and only with this mod, but when I try to connect, I get the error in the server console:

[ne.mi.fm.ne.NetworkRegistry/NETREGISTRY]: Channels [lucky:main_channel] rejected their client side version number
[ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Terminating connection with client, mismatched mod list

On the client side I just displayed Disconnected.
The server's console says I was disconnected for Connection closed - mismatched mod channel list

com.mojang.authlib.GameProfile@3c85b155[id=<null>,name=OKman65,properties={},legacy=false] (/127.0.0.1:64498) lost connection: Connection closed - mismatched mod channel list

I tried with a forge server 1.13.2, 1.14.4, 1.16.4, 1.16.5. Each time the version of forge and mod, client side and server side was the same.
The only version I tested and that works is 1.8.9.

I also have this message on the server banner

So I guess the problem comes either from a bad configuration, or from the mod that doesn't work in multiplayer above version 1.13 or maybe 1.8.9 (I haven't tested 1.9, 1.10, 1.11 and 1.12 ...).

The problem didn't come from the mod, after a good search, it came from the forge which didn't load the lucky block mod, hard to realize it when the forge error changes between versions <=1.8.9 and versions >=1.9.4.

So always check if the mods are well loaded.