Can’t login WordPress after update to WP 2.3.1

I had the problem that I can’t login to this blog anymore after I did an upgrade to Version 2.3.1. This was the case for at least a Month.

I searched the web and found some patches tried them, but with no result. (http://wordpress.org/support/topic/135598)

Finaly I tried to make the site a default as possible. Starting with the theme. And…

Bingo, when I changed the theme to the default theme, I was able to login.