[ERROR][Address already in use]?

Hello,

I've been struggling to get my ftp server configured correctly for
over a week. The following message appears in the PureFTP
Manager log (Mac OS 10.2.8):

Jan 26 21:06:39 bvk2's-Computer pure-ftpd: (? <at> ?) [ERROR]
Unable to start a standalone server: [Address already in use]

When I use Fetch, the following error is displayed:
Error: the server dropped the connection (it may be too busy).

I have FTP access ticked in the Mac OS firewall settings:
FTP access (20-21 or 1024-65535 from 20-21)
However, when I run the Network Utility program>Port Scan tab,
there are only two open ports: 21 and 139
Even when I open a new port in the Mac's Firewall tab > New > eg.
port 20, Port scan doen't show the new port is open.

FTP access is not blocked by service filtering in the modem/
router's firewall settings.

I've set up port-forwading on ports 20 and 21, to 10.1.1.2. (LAN
IP of my computer). I thought that it might work if I could change
the port to 1025 for port forwarding, however under settings in
PureFTP Manager >pure-ftpd listens on port: 21 is inactive and
cannot be changed.

Could anyone please offer a solution to this problem?