I have this problem now, on just 2 of my 5 blogs. It is very strange.
The problem is a "redirect" that is appended to the URL. In the old days of WP, when I type mysite.com/wp-admin, it will take me to that same URL with nothing else, just mysite.com/wp-admin. However NOW on my 2 broken sites, when I type mysite.com/wp-admin, it suddenly redirects to this:
http://mysite.com/wordpress/wp-login.php?redirect_to=http%3A%2F%2Fmysite.com%2Fwordpress%2Fwp-admin%2F
I will tell you that the simple workaround is: delete all the crap from the end of the URL. Go directly to http://mysite.com/wordpress/wp-login.php and it should let you log in then.
One of the blogs that has this problem is a fresh install! It has no posts, no plugins. WP 2.6.2
The exact error message is,
Request Entity Too Large
The requested resource
/mysite/wp-login.php
does not allow request data with GET requests, or the amount of data provided in the request exceeds the capacity limit.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Any ideas how to get rid of this problem?