unknown code 10009

i tried to attack world boss and got error message says "Unknown code 10009"

is this a bug ? what does it mean?

That error message occurs when you try to attack the last world boss in the set but it has already been killed. You are basically trying to attack a monster that has already been killed and replaced with a totally new set of world bosses. Just reload the world boss screen to see the new bosses.

[support7]

YES. Thans to mrmuscle's explanation.
It should be "Error: Info has changed, please reload the page and try again."
We have changed it to the correct language description. If you see similar code again, please feel free to let us know. thank u

Vicky