Thank you all for the various pointers, which helped us to isolate the problem by a process of elimination.
The solution was obvious but had to be found : php.ini does not accept 0 as value for infinity for the params post_max_size and upload_max_filesize...
The solution was obvious but had to be found : php.ini does not accept 0 as value for infinity for the params post_max_size and upload_max_filesize...