When a certain player joins, The Realm Crashes.

I Am in a realm of my friend. We were building when suddenly our other friend joined, the realm crashed. We don't know why, but whenever they join it crashes. We believe that it is an issue of the realm, or a bug. We don't know the cure and we thought maybe you could help us. If you do know what's causing the issue, please do tell us!.

The realm's Owner : Sner_