hi,
i do not know why you are experiencing that issue.
ftp with rclone is a finicky thing.
might try --dump headers --retries=1 --low-level-retries=1
fwiw,
most seedbox providers support sftp, which has much better rclone support.
also, sftp, support modtime and depending on the server, supports hash checksums to verify file transfer.
i suggest to use sftp or some other protocol.
if that is not possible, we have experts at rclone+ftp.