This may have already been said but I didn't read all the post completely.
Your user name you are log in in with might be the problem, I also had this problem until I figured it out. So when you create a client lets say "Website1" then next step is the FTP-user "ftp1" under websites. So the log in name should be "website1ftp1" for the log in to work.
You will see [clientname]"username" on the ftp user name screen. So I put them together and it worked for me