Unable to upload photo

Hi all !
In my entreprise they use Galelry3 to manage and share photos of final products inside the entreprise.
Today, the user who most often use Gallery3 came to me and say me that he could not upload photos any more.

I tried, and it seems that it is not possible (any more ?). I checked the rights on the folders var/albums, var/resizes and var/thumbs and it seems correct.
When we want to upload photos from the site, Gallery3 acts correctly (with the loading bar, the successful message and the filename appears in the list above the 'done' button) but after few seconds the filename disappears from the list and when we click 'done', the page reload but the photo is not on the album.
In the filesystem, the photo is not in var/albums, var/resize nor var/thumbs.

The server is running under Windows 2008 server R2 with IIS.

Does anyone know why we can't upload photos ?

Thank you very much by advance