Not really sure what happened. I was AFKing at my Blaze farm, and got disconnected with "Server closed" message. I can bring the server back up just fine, and other players can connect and play, but as soon as I connect, or other players get close enough to load the Blaze farm, the server crashes again almost immediately.
This blaze farm was working fine yesterday on the same version (15w45a), so I have no idea what might be different now.
Crash report attached.
Can confirm issue in 15w49b (Windows)
Edit: Sorry, found the appropriate tracker for the bug I encountered
https://bugs.mojang.com/browse/MC-93655
Looks like it was fixed in 50a, Testing with 51b and it seems to be fine.