Clumps used: 6.0.0.11, 6.0.0.10
Client used: Forge 1.16.3 34.1.17
Server used: Forge 1.16.3 34.1.17

Problem occurred only when I try to connect to forge server with forge client. I have tested some of clients and all works but forge.

Works:

• Vanila (C) - Forge (S)
• OptiFine (C) - Forge (S)

Not works:

• Forge (C) - Forge (S)

Removing Your mod fixed this, but it's not a so solution I wanted to

Nope, it's only on server. Is it wrong?

That is wrong. The mod is required on both the client and the server.

Sorry, my bad 😓

I'm super confused, is the mod on both the client and the server?

That is wrong. The mod is required on both the client and the server.

why? I can't think of a single reason for this. It's not like you would need to register a new entity... Even the old craftbukkit server supported this serverside-only...

old craftbukkit

This isn't old and this isn't craftbukkit

There may be a way to do this without an entity, but I do it with an entity and there haven't been any issues yet