I fixed the issue by myself. Modified ‘windows-azure-storage.php’ as follows.
windows-azure-storage.php @ version 1.9
before
260: $blobName = $file_upload_dir . “/” . $size[“file”];
after
260: if ($file_upload_dir == “”) {
261: $blobName = $file_upload_dir . $size[“file”];
262: } else {
263: $blobName = $file_upload_dir . “/” . $size[“file”];
264: }