alert in add histoy

hello, I have a little problem , I have 2 pages with opencart 2 a few days ago make the following settings and appeared the following problem.

change 1: a few lines added in order.php custom_field to show an attachment to the address in Print_shipping_list

change 2: the module out_of_stock_1.0 stopped working properly, stockouts buttons did not change color, the module was disabled and enabled

After the changes , the following generaro on pages

when you press the button add_history information is saved, but displays an alert (instead of displaying a message of "success" appears one alert), if you press F5 you can see the updated information

Page 1 ) displays an alert with the message of the textbox

order_status_id=2&notify=1&append=0&comment=message

Page 2) displays an alert with
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data ----
200
{"success":"Su pedido ha sido modificado correctamente"}

what try to fix it? all this not work...

1) Api invalid?
Go to admin-setings-user-api: make a new API (api2016), click in generate password and changet status to enabled and save this
after go to store-edit: go to options and change the actual api to the new (api2016) and save

2) replace the new order.php , the version before the change

3) disablet out_of_stock

but the erro continuous pressing add_history (display the alaert) It does not stop the work but is uncomfortable