If you're using an older version of Yummy FTP on Mac OS X Leopard (10.5) then you might get this problem. You should upgrade to the 1.7.2 version available from the web site, or if you still have issues then try the latest beta build:
http://www.yummysoftware.com/beta/latest/YummyFTP.dmgIf you still have no luck after trying both these versions, re-produce the issue then select 'Save FTP Transcript' from the File menu and send the resulting file to me at support : support at yummysoftware dot com