I have updated my blog to 2.7 RC1 and now I can't do login/logout from comment form.
If I'm not logged in and click "in order to comment you need to be logged in" link, I get this:
You don't have permission to access /blog/wp-login.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."
Similarly, if I want to log out the same way, I get "You are attempting to log out of [my blog name]" error screen.
Rolling back to 2.6.5 helps, but I would prefer to have this bug fixed :-)
My troublesome blog is here: http://tulentsev.com/blog/