Scenario:
Plesk server hosts mydomain.com
On mydomain.com/page.php I use a curl function to call "http://mydomain.com/page2.php"
When using PHP curl function to call a domain hosted on the same server it fails since it cannot lookup the dns for that domain.
This is quite common. The usual fix is to add the domain to the etc/hosts file on the Plesk server to avoid the obvious DNS issue.