That user doesn't look to be set up on the server at all. Did the user possibly get removed accidentally? That would be the method to reset a proftp users password(I worked for a web host company and did several of these a week for customers). You may need to re-add the user:
useradd -c 'FTP User Account' -d /path/to/ftp/folder -s /sbin/nologin
Make sure to update the path to the right folder location after the -d option. The -s specifies the shell, in this case nologin which will prevent the user from gaining SSH access but will still allow them to login for FTP.