Thanks to all,
perhaps it it's a folder permission problem.
I'll try to delete the upload folder and let the page create the new one.
@jamie: thanks for your suggestion. In fact, I've had lots of problems when there was no " enctype="multipart/form-data" in my form. But Ray already suggested to add this and now I add this code manually in every new form, expecially when there are file fields involved.
Ciao! ;)
tony