I recently updated Spigot to 1.11 on my server, and this morning I updated it because I was "19 versions behind".
After building the spigot.jar, I tried to reboot the server and this exception made me unable to restart it (see the crash log)

I have the same issue with bukkit-1.11.jar, but it works perfectly well with vanilla minecraft server.

To recreate this issue :

• put a hopper to output to a furnace
• put a hopper under the furnace
• put something in the first hopper
• the server will crash