Okay, so I know a few people have issues with this.
For now, on Ubuntu 18.04 nothing seemed to work out that great.
So I have a temporarily fix:
1. Go to your CyberPanel.
2. Scroll down to Security then >> Firewall
(Not able to find it? Enter your IP:8090/firewall/)
3. Click on the orange Pause button.
Now you are able to log into the FTP!
If you are done with your FTP tasks, make sure to enable the firewall again, this gives security to the web server. (Its important to always enable it again)
Anyways, hope this would help you!