i received that error too, and i don't know what reason behind this error
but i try this method and the update go smooth..
first Flash your Windows Phone to factory setting (setting-about-reset)
or hard reset it (when wp turn off hold down power button + volume down)
and then if the flash success you can update your phone to 8.1.
ps: your phone will update several time before getting the 8.1 update
psss: if you brick your phone when flash it, you should download stock firmware and install the stock tou your phone with NSU for Retail (you can search that on XDA)