Q: Impossible to uncheck the "encrypt backups" option

I have a problem that I can't make any sense of...

My iPhone (6 - IOS 9.3.2) was defect and i became a new one as replacement. It have never been sync'd with itunes anywhere. I would now take a backup on iTunes (12.4.1.6). The "encrypt backups" option is checked by default. I try to uncheck it and iTunes asks for a password - I have never set one. No backups have ever been done. For iCloud and iTunes Backup iTunes say that I have never take

a backup on this Computer/iCloud

I can backup the iPhone but than i need i Password that i doesn't have! iCloud is the same

So, I cant turn off encryption, and I cant restore any backups if I take one.

Anyone got any suggestions?  It makes no sense to me at all.

Thanks

iMac on El Capitan