Restore backup asks for a password, but I have never done an encrypted backup
I have never previously backed up my iPhone 6.
Two days ago I did a backup which was not encrypted, and did not require a password.
Today, I accidentally allowed iTunes to do an IOS update, which failed.
When I try to restore, I find it had created a backup this morning, which has overwritten thr backup I did, and which requires a password.
I've tried my phone passcode, my Apple ID and my Windows sign-in, but none are accepted.
Where could it have got a password from?
iPhone 6, iOS 12