https://mclo.gs/2N7Noel
Server Log

https://mclo.gs/PS4SVgt
Crash Log

When player disconnects while boomerang is thrown, it causes a server crash, according to the crash report, it appears that the boomerang entity has no where to go in its logic so it just causes a crash.

Possible fix would be to check if player is null and then kill if true? I dont know how the boomerang works, this is just my observation