Ah, here is an idea...
Try uploading the files with a relative path
/tmp/rclone-v1.51.0-linux-amd64/rclone -vv --config /tmp/rclone.conf copy /var/www/html/backup '1135:Sauvegarde/YYYYYY'
Does that work? Maybe just try copying a single file
rclone copy -vv file.txt 1135:
Maybe the FTP server doesn't like the references to the root and wants you to use the user home directory...
What do you see if you do rclone lsf 1135: can you figure out where in the file system that is?