Just typed a post for a BS thread. Hit submit and it took me to the sign in page. Went back and there's my post. Hit submit, again, and it took me to this:

400 Bad Request
Request cannot be handled by the server.

Reason: Connection with Host: header containing unknown local host name "us.bc.yahoo.com". Received on IP address (removed), which is not bound to any virtual server. Cannot proceed.

Went back, again, there's my post. Hit submit and it took, finally.

I had dumped all cookies, etc. this morning, so I don't think that's it.