@gianfrancopietraforte,
Another option is to use another plugin to do the import for you. There are several around (both free and paid). Try searching the plugins for xml import
. (Paid versions add extra features which might or might not be useful to you.)
I was able to get my old quizzes into CSV format, and then I used WP Ultimate CSV Importer to import them. It did still need some manual work, but it saved very many hours of time and frustration.
Of course, a different plugin might suit your particular needs better but, so long as you keep a backup of your quizzes, you can try out several plugins before deciding which one suits you best.