I got the same problem and have tried everything but no luck.
I had it first pwn'd alright from 2.0 to 2.0.1 custom. But then it seems that I need the no activation option to get service (I'm on Rogers). Recreated 2.0.1 custom, and DFU'd, getting my first error 6.
Okay, I read somewhere saying that I need to restore it back to the original firmware before flashing it with no activation custom firmware. I tried 2.0 but had no luck. It would not finish and gave error 1304. Realizing issues with downgrading, I was forced to put on 2.0.1 original. That worked, but now my baseband is also updated

.
Then retrying custom from this point and on, keep getting error 6. Restarted, reinstalled iTunes... just wouldn't go thru. See if someone here figures this out.
In case you think I didn't DFU correct, trust me that's not the case. DFU driver is definitely loaded each time. It just wouldn't get pass error 6.