When logging onto a server, I get this connection lost message:
Internal Exception: io.netty.handler.codec.DecoderException: java.lang.NullPointerException: Cannot invoke "net.minecraft.client.multiplayer.ClientPacketListener.m_6198()" because the return value of "net.minecraft.client.Minecraft.m_91403_()" is null.
The error appears to occur more often when more players are on the server, but still well below the limit (4 or 5 of 20.) I copied the log to Pastebin because it's far too long to put in a comment here.

https://pastebin.com/vM6Y1vLg

Same

I am also having this same issue:
Internal Exception: io.netty.handler.codec.DecoderException: java.lang.NullPointerException: Cannot invoke "net.minecraft.client.multiplayer.ClientPacketListener.m_6198_()" because the return value of "net.minecraft.client.Minecraft.m_91403_()" is null

Happens more with more players but only 2 players are currently on and I'm trying to join them.
Will very very very randomly let me in if i spam attempts to join.

ETA:
Version: All The Mods 7-0.2.45 - (Release) - (current)

I have it most of the time I'm trying to get on the server, but when I'm travelling in the end I have to restart the server constantly because it wouldn't let me join otherwise. I also crash in the end which is why I have to re-enter the server in the first place, but that's most likely unrelated.

If rejoining to server does not work, you can enter offline world, then try to connect to server. It fixed issue for me.

Bumping this issue. Still occurring.

I was also using a Jetpack (like @ErebusAres) and the console accompanied my disconnection with a "moved too quickly!" warning. Not sure if they're related.