There is a couple of related issues that might fit the bill, but they affect 4.0.1 as well:
http://issues.ez.no/IssueView.php?Id=13700&activeItem=5 http://issues.ez.no/IssueView.php?Id=14345&activeItem=12
not so related but still: http://issues.ez.no/IssueView.php?Id=14518&activeItem=6
Thank you for your answer. Such issues may be related to the problem, but anyway here the image is not uploaded at all (I can't find it the storage folder); it seems not to be a matter of wrong conversion...