Logged in but cannot access dashboard

I have upgraded from 2.3 to 2.6.2

Just unzipped the whole thing in place.

Probably should have read the instructions carefully and do something else instead.

I want to login to the dashboard as admin

I don’t get a username/password error

Username and password are correct but I get back to the login form and not to the dashboard.

When visiting the blog, I get the “edit” link for each post, which indicates that I am logged in correctly.

I have cleared all cookies related to this blog.

The cache dir is empty.

I have googled and looked in the wordpress forums for solutions but found none.

Any suggestions ?

Thanks!