Cannot change dekstop background

Hey,

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.

Can somebody please help me with this issue???

 

Hmmm..thanks....but it didn't work.
Any other suggestion. It looks like that the solid gray background on screen 1 is a layer above the originally selected wallpaper. Because when I restart the computer, the originally selected wallpaper appears when closing the machine and when starting up the machine.
Any other suggestion.