I have no idea if I should be posting this here or not, but the Wordpress support forums are not very timely in their responses.
I've been using a multisite install and last logged in a day or two ago. I haven't changed anything, but today when I tried to login, I was redirected to google.com with the following error message:
405. That’s an error.
The request method POST is inappropriate for the URL /. That’s all we know.
I have no idea how this could have happened, but it's the same no matter which browser I use.