FTP ERR_CONNECTION_ABORTED - what is the cause?

I'm testing a backup of a production server on EC2 I created from the ami of the instance. Everything is checking out except the ftp connection. The service is running, and an nmap probe shows port 21 is open. However, when I try to connect to it via a browser using "ftp://ftp2.mysite.com" as the address, I get this error:

This site can’t be reached

The webpage at ftp://ftp2.mysite.com/ might be temporarily down or it may have moved permanently to a new web address.
ERR_CONNECTION_ABORTED

I have a similar issue when trying to connect via filezilla. It successfully opens the connection, but fails when it issues the "PASV" command. Here are the last few lines from the Filezilla log:

Command:    PWD
Response:   257 "/" is the current directory
Command:    TYPE I
Response:   200 Type set to I
Command:    PASV
Error:  Disconnected from server: ECONNABORTED - Connection aborted
Error:  Failed to retrieve directory listing

The security groups for the backup and the production server are identical. I checked to see if iptables was running, and it's not in the case of both the server and the backup.

I'm having trouble figuring out what to try next. Any suggestions would be welcome.