Mac Mini :: Cannot Change Desktop Background

I've a mac mini, with two 24" Cinema Led displays. One display is connected directly to the mac mini (main screen) and one display is connected through a DVI-to-Mini-DisplayPort Converter from Athlona (secondary screen). Yesterday I rebooted the machine and since the reboot I cannot change my main screens desktop background. I tried every solution from several fora on the net, but did not succeed to change it. I only have a solid gray colored background. Normally, when you select a wallpaper, it instantly changes the background. Now it doesn't. You can see it change in the "Desktop & Screensaver" menu, but nothing happens in real. Changing the background of the secondary screen is no problem. When I restart the machine, you can see that the solid gray colored background goes away and the original chosen wallpaper appears, but then closes because I restarted the system. On start-up of the system, you can see the original chosen wallpaper again for a few seconds, while the machine starts applications to open at login, and then suddenly changes again into the solid gray colored background.