image uploader doesn't generate thumbnails anymore

Hi,

this is on an almost new blog, created one week ago with the latest wordpress. For a while I added images to my posts without any problem with the flash uploader or the browser one, and I would always get a form with radio buttons with which I could specify, after uploading the image:

4 alignment options (none, left, center, right)

4 thumbnail sizes (I found them there, by default. The largest one was always the actual dimension of the image, without scaling, and the form showed what it was, e.g 1000×500 pixel)

This afternoon, without apparent reasons, the uploader stopped generating the thumbnails. More exactly, I still see the 4 radio buttons for the different sizes, but the right one (which used to be the “no scaling, original size” choice is now preselected but shows a size of 0x0 pixels, even if the image IS uploaded properly. The three other buttons for thumbnail size selection are still there but they are unactive, cannot select them, and show no size below.

How can I fix this or at least where should I look for more info?
Server load or logs, browser configuration, what?

TIA,
Marco