Whenever an entity (player or mob) stands on a magma block, it launches a series of stacktraces about ticking entities. After we had a mob (skeleton) stand on top of a magma block, the server entered a crash loop.

Plugins on the server:

• AsyncWorldEditInjector.jar
• AsyncWorldEdit.jar
• EssentialsXChat.jar
• EssentialsXGeoIP.jar
• EssentialsX.jar
• EssentialsXSpawn.jar
• MobArena.jar
• WorldEdit.jar
• WorldGuard.jar

More information can be provided on request (not sure what else is needed).