Cannot login anymore!

Hi! Suddenly I am not able to login anymore with my usual admin password & user ID. This happened with WP 1.5.1.2 both on IE, firefox (windows) and firefox, safari (Mac). Then I upgraded to 1.5.1.3 but the problem is still there.

I tried all the password reset options in the codex. When I however type in after the error http://www.mysite.com/wp-admin/post.php things are working. When I then click on the dashboard (e.g. http://www.mysite.com/wp-admin/post.php) I get the same error…. what is going on here?

Error from firefox below:

Server Error
Error occurred:

[code=SERVER_RESPONSE_CLOSE] The server closed the connection while reading the response. Contact your system administrator.

There was an problem at the site you are trying to reach.