This seems to be happening to me while I'm trying to download from anything in the Code Geass: Lelouch of the Rebellion category - I haven't had trouble elsewhere.
I click on any particular wallpaper, vector, etc. image to download, and the next page brings up the downloadable image all right. But then when I try to right click and "save as" I get a message saying "Invalid Argument." Here's an example link where it's happened: http://download.minitokyo.net/358637 And this problem isn't limited to any one member's work. It seems to be happening to the entire category from what I can tell.
Thanks for any help, and I'll check back if there are questions still needing to be answered.