I run the latest version of Aperture on my MacBook under 10.7 Lion with a shared iPhoto library.
In addition to Owen's suggestions.
Are you switching between iPhoto and Aperture? Do you see the "unbekanntes Bildformat" after opening the library in iPhoto?
You may have a problem with the shared library. I'd try to repair the permissions and to repair the library, just to be sure it is in working condition.
Are your raw images referenced or managed? And is the library on an external drive or on an internal drive?
My Camera RAW version is 4.0.3.
Yes, I am switching between iPhoto and Aperture. The shared library is locally under /Users/Shared where my wife (with iPhoto) and me (mainly with Aperture) access it from our own user accounts. All images are managed on the internal HD, not referenced.
When I just checked it from my wife's iPhoto account, the images were okay. Only Aperture showed the black frames and interestingly considered the images to be 900x600px.
I then had to idea to select "Vorschaubild aktualisieren" (presumably "update preview image") from the context menu in Aperture and, to my surprise, it worked Didn't expect that it was a problem with the preview image because the error message suggested something different.
I will additionally repair the library as LeonieDF suggested. Thanks to you and I hope that this won't happen too often.
If you share the library in a shared folder, you probably are having a permissions problem.
Then it should suffice to repair the permission, and not the datbase.
The problem with sharing a library:
Whenever your wife is writing to the iPhoto/Aperture library, the ownership of files inside the library bundle will change. She is the owner, that is why she still can see them, but you are not.
A "Shared" folder is not sufficient to ensure, that you can both use the library safely. It would be better to move the library to a separate partion, disk, or writable disk image, with the "Ignore Ownership on this Volume" flag set. This support article explains, how set up a library to share between two users, see: