I'm not too sure what caused this crash. I talked to my two friends who were on the server at the time. One was killing skeletons in a mob farm, and the other had tamed a wolf, and was attacking sheep with it. Other than those two events, I have no idea what might have caused the crash.
Environment
Using a computer to run a multiplayer server on Ubuntu 14.04
Java version 1.7_65
Using 15w44b, but generated the world in 15w42a
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-91786. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.