For the record: after debugging with the plugin author, it appeared that the bug was in the version of PHP (5.2.6) installed on my server. The bug (cf https://bugs.php.net/bug.php?id=45862) has been fixed in PHP 5.2.7.
I have installed PHP 5.2.17 (the last version in the 5.2 branch) and all works OK now.
Thank you for your support and reactivity !
