I’m having a similar, but different problem. I also have just upgraded from 2.5.1 to 2.6. Occassionally, but not always, when I type in my user id & password, I get the password screen again, with the user id in the first field (i.e. the form has remembered it) and a blank password field.
There is no “incorrect password” message (the red box that shows at the top when your fingers slip – I know it well), just the empty login screen again. Happens with admin and other users.
I’ve tried deleting cookies, clearing the cache, using a different browser, resetting the password (emergency.php) and requesting a password reset through the GUI. None of these works.
Eventually, when I try logging in again, with the same user id and password, it works; it lets me in.
This has happened twice now. Just as I was getting ready to remove some plugins (via FTP), I thought I’d try one more time, and that time it let me in.
The ONLY thing I did the second time was delete cookies and clear cache.
Whadayathink? Anybody else seen this? Is it worth reporting this as a bug?
Cheers