If you encountered this error, you are not alone. I've encountering this on my VPS too often that I decided to use FireFTP for a while until I found out that it's a settings issue.

Here's what you need to do to solve this.

1. Go to Site Manager of FileZilla
2. Select the FTP connection where you are having problem
3. Click on Transfer Settings tab
4. Check Limit Number of Simultaneous Connections
5. Set Maximum Number of Connections to 1
6. Save and connect to your server

The issue here is that your FTP server settings only allows one active connection per IP at any given time. Of course, settings depends on the FTP server you are using so you better check the documentation. This is set by the hosting company. But if you have your own dedicated server or vps, then you'll have to check your settings.

In my case, I deliberately set it the maximum connection per IP to one for security reasons. Trouble is, I forgot I did that.