Recently I worked with FTP account which by default connects to /username folder on FTP site, whereas the filename I needed to download was /customfolder/filename.
When I put the direct link like ftp://ftp.example.com/customfolder/filename, FTPCaller couldn't download it - it looked for a customfolder inside /username folder. After a while I realized that desired filename could be downloaded by doubling the slash symbol after FTP site name, like this ftp://ftp.example.com//customfolder/filename.