Hi Ali,
Thank you for the quick reply. Unfortunately neither method was working.
I even reinstalled both JSON API, and JSON API USER plugins, then edited api.php file again.
I also tried editing two different wordpress themes functions.php, but Im still getting the same error message.
However, while I am using “Allow-Control-Allow-Origin: *” Google Chrome plugin, the issue is not present. (This is a suitable solution while developing, but not on production. )
Do you have maybe other suggestion?