Issue when importing from a private Github repository to a private Github repository

I’m doing an import from Github to Github repository, everything looks fine during the import task, the progress bar is increasing until at the end I got a "failed import " message. No details in the email, but if I jump to the new repo, try to restart the reimport and controlling it gets stuck during “Optimizing repository and pushing commits to GitHub…”, and after a while, it gives up with the fail message.
Any suggestion?