All images gone..

Somehow all images I assigned are gone and not shown with the categories.

Unfortunately the images are still in their folder on the webserver.

I deleted the images manually and tried to assign a new image to each category. I get the following message after uploading:

* files/kategoriebilder/aufstrich.jpg image removed.
* Bild hochgeladen als files/kategoriebilder/aufstrich.jpg. (in english: picture uploaded as files/kategoriebilder/aufstrich.jpg)
* Der Begriff Brotaufstrich wurde aktualisiert. (in english: the category Brotaufstrich was updated.)

After that the image still doesn't show up, but it was uploaded to the webserver!

What can I do?

Thank you in advance!