Actually Zero I think it's possible to have totally encrypted FXP between v5 and v6. The trick is you need to get flash to set the v6 to act as a client (which is what SSCN does for all transfers) or to act in receiver role.
If you do a v6->v5 which is what odd did, Flash figures out only one site supports SSCN so it enables client mode on that site, and from then on everything works great in BOTH directions which is again what appears to be happening. v5->v6 first and Flash didn't do the right thing. It looks like it tried to use CPSV on v5 which is a dumb thing to do since v5 never advertised support for the command in a FEAT response since v5 doesn't support that either. Is this the latest version of Flash? Like I said I think older versions didn't do as well as newer versions.
Also, did you try switching sides? It sounds silly, but because I think Flash interprets the client/server role differently depending on the side you queue the transfer on it really might make a difference.
For the moment just make sure you send a 1k file or something from v6 to v5 first and from then on everything should work fine
