I hope I'm not being trolled here.
It really sounds like a crappy realtek driver (or even hardware+crappy
kludge work-around driver) issue. There's no way realtek would make
windows driver updates to fix a solaris SFTP server bug- it just doesn't
make any sense.
update you NIC drivers on the windows machines or use less junky network
adapters if you have that choice.
I've never been able to take anything network related from marvel or
realtek seriously.