Cannot add product to existing order

Hi, I got problem when Im trying to add product in back office to an existing order. Here is an error what shows up when I press Add product. OpenCart V 1.5.6 and its upgraded install with theme and some modules.

Please help.
Thanks!

Nobody ? Please.

I have the same problem, using OC 1.5.6 (default theme) and get exactly this same message (except in English).
"Missing argument 4 for Cart::add()..."

Does anyone have any ideas?
Thanks!

We don't see error message, but if we add a product to an existing order that has 7 items with 6 items being free, after the status bar says processing, and a green bar saying total has been updated, the new addition does not show up in the list. We tried using different browsers and the result is the same. We have already checked other tabs that the shipping address details have all fields marked with asterisk being filled out.

i have solved our own product addition problem. The cause is that the product status is disabled. So after we change it to enabled, the addition works.