First of all, FlashFXP v2.1 b924 is an old version. You should upgrade to v3.2 (the latest non-beta release available at the moment) before doing anything else.
You can download it from here:
http://www.inicom.net/pages/en.ffxp-download.php
Then, are you also using a firewall or a router ? If so, please tell us which ones.
http://www.inicom.net/forum/showthread.php?t=4745