Lately I was asked to get FTP working on a ziggo ubee modem (new type with wifi). All ftp sites were working properly but the connection got stuck on opening folders. Turns out Ziggo delivers the modem with Firewall on Medium. This setting is blocking all inbound traffic when it's not in the list (list is not configurable). When you set security one step lower the FTP works like a charm. Strangly enough Ziggo didn't solve the problem and wanted to send a engineer on the costs of the customer to fix it.... Shame on them cause it was one mouseclick that enabled FTP again.