Uploading image fails – without error message

Hi,

I’m having a problem uploading images.

I select image, it uploads, and all appears well. No error message.

When doing an “edit” the image shows just fine.

However I can not get a thumbnail to show in the gallery, I can not add them to my posts, they do not appear anywhere on disk (at least not in the /blog/wp-content/upload/ directory: that one is empty). It is stored somewhere, as the edit function shows the image! It simply never shows up, except in the edit function.

This is a new installation on Debian.
I checked permissions, can’t find a problem there.
I don’t have any plugins running and have tried both the 2011 and 2012 standard built-in themes; both give the exact same result.

Any ideas?
I’ve been looking at many other posts but nothing that gives me a solution to this highly frustrating issue.