The current version of lftp in the debian repository does not connect to an own sftp server,
using self genereted certificate. It simply hangs at the ls command (list directory) upon connection.
By starting it using: lftp -d you can see if this is the same problem on your side.
The reason is because on first connect, the server is not yet in the list of known hosts.
To circumvent this issue, there is a simple workaround:
Connect to the server via ssh, like:
ssh username@server.url -p portnumber
Upon request to add the certificate to the known hosts file, enter yes (only if you know you are connecting
to the correct server - verify the certificate).
lftp uses the the known_hosts file of the current user to perform its validity check.
After the certificate has been added, you can connect as usual:
open sftp://user@server.url:portnumber