Can't update product in admin panel

[Czadus]

I have a weird behavior and I don't know where it comes from. The shop is not something I implemented, I just want to help my friends resolve the issue.

The problem is that when I try to update product in admin panel, response looks more or less like this:

test{"product":{"tax_name":null,"tax_rate":null,"id_manufacturer":null,"id_supplier":"0","i....................

Post goes to this url: /index.php/product/form/136?_token=..........

So as you can see there is test at the beginning of the response and I don't know where to look foor it. I found echo 'test' in classes/Configuration.php but that seems to not resolve issue. So any idea where to start?

Edited February 3, 2019 by Czadus (see edit history)