A broken entity or block crashed the world
What causes it
One entity or block in the world hit a bug in a mod and crashed the server while ticking.
How to fix it
The crash report names the entity and its position. Restore a recent backup, or ask support to remove the broken entity from the world.
Usual suspects: complex mob mods (Alex's Mobs, Mowzie's Mobs, boss mods); tech and automation mods (Mekanism, Create, Powah); magic and ritual mods.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log