That's the same problem I have been having. The only thing that I can find that's weird is a "T" argument at the end of the command string that Idk what It's for. I really wish I could get it to work but just haven't had the time to dedicate a night to making something work when cgminer works fine.
You get 15Kh/s with bfgminer versus 500Kh/s with cgminer? If so
please email me. I've asked several times for people to speak up if they get better scrypt performance with cgminer so that I can look into it.
The T argument just makes cgminer / bfgminer give text-only output.