I was running lftp with the -f switch to launch the usual mirror command, working fine with a few ftp servers at first, but when I added another sync with a pure-ftpd server something stopped working as intended: the message
source: Is a directory
appeared and nothing more happened, only after I pressed Enter the script went on with the execution.
What helped me was this discussion.
so I changed my script, and instead of having
user username password
open -u username,password host
and everything was magically fine!
Why? NO IDEA. Who cares. Works now.