I installed a FTP server called vsftpd on my Ubuntu Server recently. After the install, I’ve been unable to login at all. It turned out to be a bug within the vsftpd program. I found a fix here. Essentially, the fix was removing the vsftpd program first, deleting the /etc/pam.d/vsftpd file and forcing the application to recreate the file. The third step is to reinstall the vstfpd once again.
From the Terminal, type the following commands.
sudo apt-get remove vsftpd
sudo rm /etc/pam.d/vsftpd
sudo apt-get install vsftpd
After making the changes, I was able to log in successfully using the Filezilla client. Success.