I've seen this crash a few times in the past week, and I just managed to catch it in the debugger. I'm not 100% sure what's going wrong, but I created a patch for what I think is wrong and have deployed it to the server. What I do know is that the crash is triggered by a certain situation when leaving an age. I still have the server running under the debugger and will be watching... If the shard stops responding, let me know! That means I've got something.
On the plus side, in looking for this crash, I discovered and fixed a problem in the server that could allow (skilled programmer) malicious users to crash it.