Packets are arriving in a shape the server cannot read
What it looks like in your log
io.netty.handler.codec.DecoderException: java.lang.IndexOutOfBoundsException
What causes it
A DecoderException means one side wrote a packet the other could not parse. On a modded server that is nearly always a mod present on one side only, or the same mod at two different versions; behind a proxy it is double compression or a mismatched forwarding mode.
How to fix it
Match the client and server mod lists exactly, version for version, and start with any mod that touches networking or rendering optimisation. If it happens to one player only, it is their install; if it happens to everyone at the same point, it is the server or the proxy in front of it.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log