Hi! This happened to me. I'm using Velocity with Forge Server 1.20.1. I have no plugin installed on Velocity and some mods on servers ( Proxy Compatible Forge, The Man from The Fog, GeckoLib ).
When I try to join servers through proxy it kicks me with the error:

Netty Epoll Server IO #2/ERROR] [ne.mi.ne.NetworkRegistry/NETREGISTRY]: Channels [man:man,geckolib:main] rejected vanilla connections

Some logs here:
Velocity: Here
Forge Server: Here
Velocity Config: Here

This server has mods that require Forge to be installed on the client. Contact your server admin for more details.