Hi Pigui and thanks.
I had managed to restore the backup I sent you without any problems except that I didn't get 13 wikis. I got fewer wikis and all of them appeared as if they had been created by me. But I didn't edit any of the pages (except for maybe a title in the 'first page' of some of the wikis). The students wrote all the wikis.
I don't know whether the problem is in the backup or in the restore, really. If this had happened a few months ago, I would have thought that the problem didn't have to do with NWiki because we had some problems with our installation. But now everything is newly installed and we haven't had problems with anything else. Backups and restores are the weak point in Moodle or at least with respect to NWiki and perhaps other modules.
Do you have this restored course anywhere where it is accessible? If you do, send me a link via a private message. I'd like to see these 13 wikis and try to figure out who they belong to.