After upgrade to 1.5.5a admin login no longer possible

After upgrading to 1.5.5a from 1.5.4 it was no longer possible to log in to the site admin. entering what should have been a correct username and password resulted in the login screen being brought back up.

The 'forgot password' link takes you to a form to enter an email, but I am unable to find any evidence that an email was sent.

There is not a failed login recorded on the admin account record in the database. There is no log in the zen-cart log folder, and there is no error in the server logs...

any ideas? Any specific information needed that isn't included here? I know an error would be great, but there doesn't appear to be one recorded anywhere...

When there's no message on-screen saying anything about a problem, that typically means PHP couldn't start a "session" (which is how it remembers you from page to page after login).
Often this is caused by damaged cookies in your browser, a changed directory path to your store, or a misconfigured server.
This FAQ may help: I can't stay logged in to my Admin area -- or -- PHP Session Handling isn't working for me.

Don't know if it is connected but the first time I wanted to log in to my upgraded 1.5.5a it would only work with the Admin user name "Admin" - then it ran the Db update (only) and after that all the Admin names worked?