API is adding whitespaces at beginning of JSON response

Hi,

I’m having some troubles with wp-json api responses. WP don’t let me add new images, showing a message that the json response is invalid.

The image uploads correctly to the server, but the page don’t take the uploaded file.

I debugged the response, and I see a lot of whitespaces (red dots in Google network inspector) before the JSON. Any idea of what’s going on?

I’m using WP 5.1.1

Thanks!