// open the connection to start sending the file

		if (!preg_match('#[0-9]{1,3},[0-9]{1,3},[0-9]{1,3},[0-9]{1,3},[0-9]+,[0-9]+#', $ip_port, $temp))

From line 738 of functions_transfer.php

This regex will only match an IPv4 address. The other portions of phpBB support IPv6 addresses.

This looks like a complicated fix due to the way the port number is tacked onto the IP address. Maybe the port could be prepended instead of appended to keep it relatively simple.