I have a private multiplayer server, and whenever I fly around with a elytra and someone is in the render. It will CRASH there games completely. This will only happen if the player is in the render, they have a lunar client cape on, and a regular Minecraft cape that you can change from the Minecraft launcher. I have fixed this issue by taking off my lunar cape.
I know this is true because not everyone plays lunar client on the server, and there game doesn’t crash.
So the problem is only when you have a default Minecraft cape(In this case the cherry grove cape) and any lunar client cape over it. Please fix this