550 File or Directory Not Found

I am at a loss to understand the mechanism by which I have run into the following problem...any recommendations for a fix will be greatly appreciated.

I am running SmartFTP 3.0.1010 on a networked XP Home desktop system, Smart FTP 2.5 on a networked XP Home laptop, and FileZilla (freeware FTP server software) on a networked XP Pro Tower.

I am able to send and receive files to/from my FileZilla FTP server software running on the XP Pro system from my notebook running SmartFTP 2.5, but not from the XP desktop system running SmartFTP 3.0.1010. My user accounts and settings are identical on both the XP laptop running SmartFTP 2.5, and XP tower running SmartFTP 3.0.

The XP tower running the SmartFTP 3.0 version gets the "550 File or Directory Not Found" error message when I try to uplopad files.

If anyone has any suggestions as to how to explain this, I will be greatly appreciative!

Thanks so much in advance for any help.