this is very strange problem.
is the ftp server using any type of ftp over tls?
i would remove port": "21", from the config file, no need for it there.
https://rclone.org/ftp/#limitations
i would read that section, about the debug flags and test with them.
perhaps this might spark an idea?
Error reading source directory: 425 Can't open data connection for transfer of - #5 by Animosity022