iPhone 6s backup and restore from iTunes
Thank you for making sure to note all of the iOS versions and that you updated to 10.3.1 already, since that would definitely be my first suggestion.

I work on software called Decipher Backup Repair, that deals with issues just like this. Sometimes there are glitches in the backup that cause it to be corrupt, and we fix those issues to make a version of the backup that is restorable in iTunes again.

Before getting to our software though, I would double check a few more things, such as trying a different cable, usb port, etc, just in case, if you have extras handy.