Iis – Why can’t I connect to this IIS FTP

I have a Windows 2003 server where i've configured FTP in IIS.

I'm by no means an expert in FTP configuration, and I don't recall any special settings I could've made.
This worked initially, but at some point it just stopped working, and I can't think of anything that could've changed that could've broken it.

Here is the log that FileZilla is giving me:
(censored to protect the innocent)

Status: Resolving address of www.xxxxxx.com
Status: Connecting to xx.xx.xx.12:21...
Status: Connection established, waiting for welcome message...
Response:   220 Microsoft FTP Service
Command:    USER xxxxxxxx
Response:   331 Password required for xxxxxxxx.
Command:    PASS **********
Response:   230 User xxxxxxxx logged in.
Status: Connected
Status: Retrieving directory listing...
Command:    PWD
Response:   257 "/xxxxxxxx" is current directory.
Command:    TYPE I
Response:   200 Type set to I.
Command:    PASV
Response:   227 Entering Passive Mode (xx,xxx,xxx,12,4,44).
Command:    LIST
Response:   425 Can't open data connection.
Error:  Failed to retrieve directory listing

I've tried setting FileZilla to connect using both Active and Passive mode. Neither works.

I'm not sure what could be going on… I've configured a number of FTP servers in the same way, and they all just work.

Any ideas?