It was really very simple: I formatted the server, I installed the dropbox extension again, logged in with my data, and I found my backup previously done.
Very important: in plesk, in the backup security settings you have to enter a "password specified" that will allow us to import the Dropbox backup in any distribution with plesk.
Then when you have to import, select the backup file from the dropbox, choose the one you want to import (db, files, ssl certificate ...) and flag "Restore this backup regardless of whether it has a valid signature"
Et voila