Afak this is problem with FFXP. What really happends is:
CLIENT: PORT a,b,c,d,x,y
SERVER: 200 PORT command successful.
--- HIDDEN
CLIENT: SIZE filename.ext
SERVER: 550 filename.ext: No such file or directory.
-- HIDDEN
CLIENT: STOR filename.ext
SERVER: 550 filename.ext: Permission denied.
^^
This error comes 'cause file already exists in directory.
Both size command & reply are hidden; for some reason ffxp gets the previous error message, instead of the current error message. (This might be fixed in the latest build) You might want to check flashfxp forums:
http://forum.flashfxp.com/ for further information about subject.