A friend with a linksys router has just told me a couple of things that he's found out when fxping. Whether this will help some of you I am not sure but try it:
(1) In site manager - enable the option 'Site Uses IP Masq/NAT/Non-Routable IP' on the options of the server that you are FXPing
TO (Not from) and also enable the alternative FXP Method 'Uploading (PASV)' - although i'm not sure if the 2nd option is critical.
if that doesn't work try using the 'Send NO-OPs during transfer' option in the preferences 'Proxy / Firewall / Ident' screen.
if that still has the same problem - you can try adjusting the frequency of the no-ops being sent by adjusting the SKAI=60 setting in the FlashFXP.ini file in your program folder. As you can see mine is set at 60 (seconds) and I now have no problems fxping whatsoever.
Word of caution though - some ftps do not like the no-ops option so use with care. You could be banned for sending no-ops too frequently.
Lemme know if this solves anyones problem so I can feel satisfied
Del^