No preview images for slider on theme administration interface in wordpress

Hi,

I'm using the Theme Architecture for a website and have encountered the following problem: there are no preview images on the slider configuration interface, found under

<theme name> >> slider

Initally they were there at the first time you uploaded the images and left blank after a reload. Since the update to the latest wordpress release, they are not shown anymore at all.

The slider itself works and does also integrate newly added images.

It is just very inconvenient to use, without a preview picture.

I read, that the preview pictures should be found under:

/htdocs/wp-content/uploads/<image name>_prev.jpg

along with all the other used images, but they are not there. The "uploads" folder permissions are set to 775.

Some information about the site:

Used Theme: Architecture / activated
Wordpress: 3.7.1 german
Plug-Ins:

1) Kommentare deaktivieren ("deactivate comments")
2) Facebook
3) Formbuilder
4) Issuu embed
5) NextGEN Gallery by Photocrati
6) PanoPress
7) Use Google Libraries

FTP-Access works without a problem, regular FTP without SSL.

thank you very much for any help in advance,
Bernd