Did you run it through the validator, as suggested in IMS_Common_Cartridge_import_and_export?
Eduardo, you probably figured out if you followed Howard's link to get the validator, but "Moodle Documentation leads me to an imscc validator tool that no longer works." Thanks to Pamela who reported this broken link May 2018 last year (and, yes, I tried it again & the link is still broken today). Sorry, I'm having trouble with restoring my .imscc backup, too, so I can't be much more help.