the "Ohne Wertung oder besser" setting in the Browser is filtering - this will not show rejected images. To be completely sure, set it to the german equivalent of "Showing all" (probably "Alle anzeigen").
In some rare cases you can have an invisible filter, see Frank Caggiano's User tip here:
Apart from that, I'd start with the Aperture 3: Troubleshooting Basics-
Try first all three library repair options, then removing preferences. A corrupted library may prevent library items from showing, also broken preference files.