action/login 404 error

This is a new problem with the update from 1.6.1 to 1.6.3. When I attempt to log in from the main page, the server returns a 404 error: "The server can not find the requested page: mysite.com/action/login.php (port 443)"

The login.php file is definitely intact in the actions folder. What gives?

I do have vazco_topbar installed, which has its own login box -- still working. I'm reluctant to disable it to check for conflicts, because that might leave me with no way to log into the site!