A crash report
crash-2016-11-19_18.46.48-server.txt

Did that only happen once or is it a recurring thing?

Did you change the config for the chance of Lush Caves?
If you want to turn them off, there is a seperate config for that.

It happened just once. I changed the spawning option to false so maybe that will fix things. I'll let you know if it happens again.

You don't have to turn it off. But the issue happened because of a config option which, if you didn't change it, is pretty weird.
I'll close this for now, but if you manage to reproduce it, please re-address this issue!