I have had this problem for the past 2 days and I finally fixed it. It had nothing to do with the system but the the connection between the battery and the phone had gotten messed up. It kept trying to restore but only ended up with error 21.
My solution: I opened the phone, removed the battery, cleaned out the battery connector and reattached the battery. The phone turned back on and showed the low battery sign. I had done this before and it didn't work. Apparently the problem was I hadn't properly connected them. You need to make sure it's locked on tight so when you close your phone slides on easily. I hope this can help someone else. I was ready to buy a new battery and now I don't have to.