Cannot reset lock screen password
Assuming that there's not an issue with the password being correct, I'm wondering if there is a chance you set encryption on the device. 

On my Samsung, that forces the password to be an actual password, as opposed to a PIN or swipe. In order to change the type of screen lock away from the high security password, you need to first decrypt the phone. It should then let you select the screen lock of your choice. Worth a shot to look!