big problem in opencart

Hello every one My name is Mohammed
I am already install opencart verion 2.0.0.0

all thing is okey , when the custmer add new order
and I want to update the status of order for exapme from "Pending" to "processed"
and click to "Add History"

I got this message I do not why ? and what is the problem
if there anyone had this problem and soloved it please let me the answer

Thank you so much >!

I find this sort of error usually indicates a file didn't upload properly to the server. I would reupload the files (other than install directory and config files). If it gives you a specific file which by what you posted, didn't, I would upload only that. You might start with the javascript files though and see if that solves it.

It seems the error occurs when site is in maintenance mode When in live mode is seems to work!

Mark as [SOLVED] in the title if it works for you.