Problem solved.
1) xmlrpc doesn’t work coorrectly with the staging area (a clone of the production environment) function offered by Siteground. I created the staging area today to discover why Jetpack wasn’t working on the main website and i disabled all plugin in this test environment but not on the main one
2) Siteground tech staff solved the problem on the production environment: it was a plugin, Iubenda Cookie Law Solution. With this plugin disabled i can succesfully connect Jetpack that continue to work also when i re-enable it.
Thank you for your help