GitLab webhook error 'Hook Execution Failed'

I have a vbox with a virtualhost running a website which I can access through the host. That website is on a gitlab repository, which is running on a local server.

I wanted to setup a webhook so that when I pushed on the host, it would pull on the vbox.

I already have a php script at http://192.168.1.1/webhook.php and I've tested it with curl and it's working just fine. But when I input that URL as a webhook URL on my gitlab project and test it, it shows a 'Hook Execution Failed' error.

I have also tried:

1. setting up a webhook with a http://requestb.in/ URL and it worked.

2. running the website on a virtual host on the host machine (not on vbox) but it did not work.

3. typing the url on the browser manually http://192.168.1.1/webhook.php or localhost/webhook.php (not on vbox) and it did run the script.