Occurs when a player is generating chunks. A region fixer reports no chunk errors. It's not easily reproducible, but it happens while a player is generating chunks. The world is over a year old (although the server is semi-private so not explored so much).

https://gist.github.com/RoboMWM/19d21c55c5a7402409df

History of other crashes: https://www.spigotmc.org/threads/possible-chunk-generation-error.97403