I recently launched a server and everything was going fine. Eventually, my friends and I decided to explore farther out from our home. After a time of exploring, someone ran into a chunk that would just crash their game. When they rejoined, the same problem would happen. The only way was to teleport them out of the area as soon as they joined, which gets a bit annoying now. We started to avoid that chunk until we came across more in different dimensions. The problem just keeps growing after more exploring.
The Crash Log http://pastebin.com/PtShAGSG