I've come across this twice now, so trying to figure out what's going on as it will a rather common task for me.
I have a site developed and live on a *Nix server. Working fine.
I have since decided to a local copy to a new computer running windows and WAMP for development purposes and so I can upgrade the site from 2.2.3 to 2.3.0.
I have transferred the code base (and updated sapphire, cms and jsparty to 2.3), and also transferred a copy of the database with MySQL lower_case_table_names set to 2 so all the table names are matching between local and live.
The local site appears to work fine aside from a few js errors, however I am unable to login to admin using the same credentials that I use on the live site.
Any ideas of why this would be? As I said, this has now happened twice. The first was a direct copy of a 2.2.3 site, the second was a live 2.2.3 site updated to 2.3.0 locally.