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:
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
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?