Field container = ContainerAnvilInventory.class.getDeclaredField("a");
(From MainAnvil.java)
this throws an exception every time the anvil is used, since i believe "a" is a private field
i also saw your comment there, figured you had some issues anyway

That says you're using 1.8.6 for a server, which I haven't built for

What server are you using?

It appears to be running the 1.8.1 code for some reason. I've tested on my own Spigot 1.8.3 and it worked fine. Try getting a later 1.8.3 build?

http://getspigot.org/