I was getting error 21, error 1600, and error 1601. But I finally got it working. For me it was a two-pronged issue.
Error 21 points to bad cable, or bad ipsw.
First, let Seas0npass create the custom firmware, but don't worry about it automatically restoring with it's scripts, they failed everytime for me. After I had the custom firmware created, I placed it on my desktop. I then unplugged and replugged my atv2 back in, and let itunes recognize it. I then performed a FRESH restore with the official apple ipsw.
Once it's done it will say plug into tv. Unplug it from your machine, wait 5 seconds or so and plug it back in. itunes should recognize it again, then search and manually restore to the custom firmware. I've done it a few times now, and if I don't follow this process I get error 21. fresh restore then custom.
you may need to put the atv2 into DFU mode right before you do the custom firmware (plug in wait 5 seconds till light starts flashing rapidly, then menu and play for 7-10secs.). i've been able to do it NOT in DFU mode a couple times, just right after a fresh restore. another couple times I had to have it in DFU mode right before the custom restore. odd, but i promise I've done it both ways... following the process of fresh official restore, then custom back-to-back has solved my 21, 1600, 1601 errors.
If this still doesn't work and the custom restore starts but fails after ~5-10% with error 21 it might be your cable. i had this issue as well. I was trying to restore the custom firmware, it would get maybe 5-10% and fail, error 21. I decided to use another cable (oddly enough a Blackberry micro to usb) and it worked. so it might be your cable, try another cable. I purchased one online, and it's a pos, I can get it to work everytime with the BB cable, but not with the one I purchased on Amazon.
I've done all the restoring in a OSX VM via Win 7, so I'm positive those with a macbook should have fewer issues and this should resolve your error 21. i've then been thetherbooting in windows via tetheredboot utility. (see tether boot issues post).
try a new cable, and try the back-to-back restores with the offical and then the custom.