I manage my company's devices, I regularly restore iPhones with iTunes on my PC. I had restored a couple of iPhones when I met this error on a new 11. I tried everything (to the point of near-madness) and I called Apple for guidance. They could not help me (they don't exactly seem to know what provokes this d****d error, which is mind boggling).
I tried on another PC at work, same problem. So they suggested that it might be caused by our company security protocols (I find it strange, considering I can restore most iPhones without a hitch, and have always been able to, but... why not I guess).
I then tried on one of our graphic designer's Mac and... it worked !
I opened Finder, did the usual choreography on the device to restore it (buttons and such), clicked on Restore and Update on the Mac and (hurray!) the iOs update started downloading, as it should.
This is the only solution I could find. It is NOT practical as it requires a Mac but it does work. If this error happens too often I will ponder the thought of a possibility of maybe buying a Mac just so I can restore the devices myself without bothering my colleagues in other departments or having to go to the Genius Bar (IF my superiors allow the ridiculous purchase).
for those who don't have access to a Mac, I don't see any other way than to get to a Genius Bar (depending on where you live, this could be easier said than done, for sure).
Good luck to you all !