I have bought a forge 1.16.5 server, a single person can be online but when someone else joins the server is frozen (for them), until they're fully loaded in, then it kicks them and says, "Remote Connection Closed by the Host". The player that was previously online can see their character loaded and the server announces they've joined. This problem has persisted throughout different hosting sites, even locally hosted on my pc.
Here is the mod list:
and the SERVER crash logs don't indicate anything wrong with the player, it just mentions their client has disconnected.