Restore default_server page Nginx

[MattiaM]

Hi folks,
I have a problem with Nginx.
I want the Default Parallels Plesk Panel Page when something goes wrong with the client request.
I can't understand how force the default_server page in Nginx.
Nginx has changed this parameter seems without a logic...
I tryed to change manually the configuration but it doesn't works...

Thansk in advance to all....