I have determined that this mod is causing our server to crash after analyzing our crash reports and looking into the mixins involved. The functions this mod calls in the spawnBalloons method clearly matches the stack trace.

I have attached my crash log.
crashlog.txt

The crash you have sent doesn't relate to the spawnBalloons() method.

The last method called in the stack trace that isn't a writing operation is func_237335_a_().
func_237335_a_() is PlayerData#savePlayerData(), which means this crash happens when a player's data is being saved.
When a player joins a server, the spawnBalloons() method spawns the balloons that were attached to the player.

Therefore, it cannot be the fault of the spawnBalloons() method because it's a method called during connection, and the crash is happening during disconnect.

Does the crash no longer occur when Endergetic is removed, and when does this crash happen?

Unfortunately for me, the issue was not in relation to Endergetic. The server ran for hours without issue after it was removed, until it crashed with the exact same issue just recently. My only guess is that it is being caused by the other mixins it's referring to in the stacktrace. Furthermore, in the majority of the crashes linux steps in and terminates the process before a crash report is generated.

Closing this issue due to not being related to this mod.