I just upgraded the 3 blogs hosted here on my site to WordPress 2.6, as well as a number of other blogs I maintain for people.
In each and every case, after upgrading, I had some trouble getting back to the Admin page – the login page appears and it says “Please log in again” in pink above the login prompt. (Sadly, I didn’t think to get a screenshot.)
I did some digging and found a suggestion that clearing your Cookies would help – and indeed it did, for all the sites. Whew!
I do love this version of WordPress (as I love all versions – WordPress is great!) but it is a rather odd thing – it scared me at first, let me tell you! I thought something had gone horribly wrong or that my password had somehow changed. But it was just some weirdness with cookies.
I’m posting this here so that other people can find the answer – it’s simple enough. It seems as though there were some changes to the cookies used by WordPress in this version – perhaps that’s what caused the problem?