There are a lot of simple things which I usually forget how to do it again. I will post them under "I Will Forget" title and "iwf" tag.

You access a new pretty Windows Server with slick Metro interface and not some black and white console. You want to transfer files to that server manually via FTP. Then you download and install FileZilla server edition. Unfortunately, you can't access it from your FTP client. The message is "Error: The data connection could not be established".

First, ensure port 21 is open. You can also try to set firewall to allow FileZilla server program. If your server is hosted on EC2, ensure your security group doesn't block it.

Somehow your FileZilla server shows warning that your server is behind a NAT router.

You need to set up Passive mode settings. Set IP there to point to your server's IP and specify allowed ports.

Then you try your FTP client again. It says it can't list available directories.

Probably you forget to unblock the allowed ports in passive mode ?