Multiplayer does not work in Valkyre Angel Boss

[GttiqwT]

I keep having this issue with the Aether mod. When me and my friend get to the Valkyre Angel boss we cannot beat it as it keep kicking us and throwing random errors on the server. The server does not crash but we cannot connect until I restart it. Once done it'll let us log in once and then after going back to the boss room it'll repeat. Either saying "Disconnected" or "Timed out" or some sort of java.util.lang or java.util.IOException error. It says this in console
[01:10:58] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class com.gildedgames.the_aether.entities.util.EntityMountable from class com.gildedgames.the_aether.entities.util.EntitySaddleMount

Please fix as its unplayable and just breaks/locks up the server. Thanks