Getting that same problem? Well apparently you are not alone, seems ProFTP and ClearOS don’t really work well out of the box. FileZilla would constantly timeout on listing the directories, so it would never actually connect. Transmit would timeout too, but it would spit out different errors, usually Error 162 Port Fail. After hours of digging I did find the problem, and the solution. It was actually pretty easy to fix (makes you wonder why the fix is not implemented?). Depending on your ClearOS setup YMMV and other steps may be required. I am running my ClearOS box in gateway mode, it is my server and router.

Start by forwarding all those ports required. Clear uses different ports for different things with FTP.
21 standard ftp port (internet say this is for home folders, but it connects me to flexshares). 60,000-61,000 are used for port 21 PASSIVE connections 2121 standard ftp port (internet says its for flexshares). 65,000-65,100 are used for port 21 PASSIVE connections. 989/990 SFTP ports.

To ditch the unroutable address warning toss this in your proftpd.conf file:
sudo nano /etc/proftpd.conf MasqueradeAddress ftp.mydomain.com  # using a DNS name

This did allow me to finally connect with Transmit with no issues. A friend of mine was able to connect with FileZilla after I made the changes. I had to change my FileZilla settings to use an Active connection versus a Passive or mine would refuse to connect. Odd.

I found these sites to be helpful, and thats where I got my information.
http://www.proftpd.org/docs/howto/NAT.html
https://www.clearos.com/clearfoundation/social/community/remote-ftp-access-through-nat-router-solved
https://www.clearos.com/clearfoundation/social/community/ftp-problem
http://forum.slicehost.com/index.php?p=/discussion/4491/ftp-error-162-port-failed/p1
http://www.theserverpages.com/articles/servers/cpanel/tweaks/Getting_passive_FTP_connections_to_work_through_a_firewall_properly.html
https://forum.filezilla-project.org/viewtopic.php?t=7315