POST return 403 forbidden

I have a few IOT devices that can trigger a GET or POST http request when some events are triggered.

I wrote a small PHP script on a page of my OVH hosted site to handle them.
My GET commands are properly accepted and managed.
However the POST /json are not reaching the script and are returning a: 403 Forbidden error.

What I am missing here? I don't believe I have any limitation on my .htaccess. I also have no firewall. I am sure it is something very simple but I haven't found it as of yet.
So if one of you could lend me a hand by pointing me in the right direction I'll appreciate.
Thanks