What language / kb layout are you using?
When I thought about that I realised that the password had a character in it that is in a different position on British and US keyboards so I:
1. Set a new password that doesn't have that character in it (using sudo su - I had added my user to sudoers using a separate boot) and that worked.
2. Then again using a root terminal I changed it back again to the original one. And to my surprise that now works also.
So I think it is possible the installer in 8 may have some confusion about the keyboard layout. After all I have been through a number of installs of it that have had the same result with the same password. I do specify British English in the installer FWIW.
Maybe I am the only person who uses such a character in passwords && has British Keyboard && installs Debian 8 from the live DVD
For some reason, resetting the password via grub did not work.
But changing it in a root terminal eventually did.
Anyway it now works, and there is even an hypothesis to explain what happened.
So thank you