I'm seeing a lot of this:

[11:47:59] [Netty Epoll Server IO #8/ERROR] [minecraft/ArgumentTypes]: Could not serialize codechicken.lib.internal.command.EntityTypeArgument@364a170a (class codechicken.lib.internal.command.EntityTypeArgument) - will not be sent to client!

Server 1.16.4
Forge 1.16.4-35.1.0
CodeChickenLib-1.16.4-3.5.0.398
ChickenChunks-1.16.4-2.7.0.85
EnderStorage-1.16.4-2.7.0.159

Fixed in 985e7fb fixed build on maven is over here. However its safe to ignore this message.