My server stopped allowing users to login to FTP through port 21 and only allowed port 22 and SFTP.
After a lot of digging I found the solution by loading the FTP connection tracking module on the server.
This can be done by logging into Putty and running the command:
I found this answer on this thread: http://www.cloudmin.com/node/34246
An it pointed me to this Virtualmin Documentation: https://www.virtualmin.com/documentation/web/faq#FTP_Service_isn-t_working
Hope this helps everyone.