OK, I'm back now and I tried to restore the backups using BackupMenu.

With a fresh flash of the RootFS followed by the reinstallation of BackupMenu from the repositories, the two backup files were found in the restore menu. I first tried to restore only the OptFS. It was successful according to BackupMenu, but when rebooting the N900, I saw absolutely no change (same applications as before, that is to say almost none). So I tried to restore both RootFS and OptFS using the adequate option in your application. I don't know if it was successful or not, I was not watching the screen during the process and when I came back to check, the N900 was shut off. Now, I cannot boot it, it just displays the white Nokia screen and then switches off. The same thing happens when booting with the keyboard slided out, so no way to come back to BackupMenu. The battery was full.

What really annoys me is that I can't flash again using Maemo Flasher. I flashed my N900 many times and this is the first time I can't do it. Here is the error I get:

The previous step all work until 100%, but I can't copy text from my command prompt so I juste wrote here the error lines.

Is it bricked? :/

[Edit] OK, I fixed the flashing issue by flashing without the -R option. It worked and now I can flash flawlessly again, with or without -R. But this doesn't really say us why restoring OptFS did not actually restore anything and why restoring RootFS and OptFS screwed up my N900. I'm going to reinstall apps manually and create new backup files just in case, but I'm really interested in your app so I would like to understand why it was not working in my case (and only in my case, obviously). Maybe the backups are corrupted but it would be surprising as BackupMenu said "Success" for the OptFS restoration; and I see no reason why they would be corrupted.