iTunes 12.3.0.44 (x64) on Windows: can't backup iPhone 6 "because it hasn't been configured yet"

Hi Everyone,

I'm in a bit of a pinch here: since Apple decided that iPhone 6 and above needed iTunes 12.x.x to sync and backup, I'm down to one computer for backup and syncing, namely a Lenovo laptop running Windows 10; my faithful MacBook Pro circa 2006 can only run Snow Leopard (it's 32-bit only), and can't therefore accomodate more recent versions of iTunes than 11.4.

Today, I've plugged my iPhone into a USB port on the PC and tried several times (using different ports) to sync it (that works, sort of, in the sense that the sync button is operable) and back it up, but that won't work, because, according to an error message, "[the phone] hasn't been configured yet".

Like I said, I've tried every trick in the book, like switching USB ports, restarting the PC with the iPhone plugged in, etc...

Nothing works; I can sync the phone, but the date of the last backup is three days old, as indicated in the iTunes main window.

Ideas, anyone?