can't erase iPhone to sell it - Apple ID password not accepted

I am trying to erase all the data & settings on my iPhone 5S in preparation for selling it.

Here's what I'm doing:

Settings

General

Reset

Erase all data and settings

confirm it twice

Enter my Apple ID password - which is, by the way, correct (double checked it and here I am, all logged in)

So, what to do? I have 2 iPhones to erase/restore in preparation for selling them and this is not helping.

The phone is actually using OS 9.1

Thanks for all help.