I'm having the exact same problem. I have password protection enabled and within the past few days, something has changed dramatically. The login box that used to appear prior to accessing the site is no longer there, just an unusual SSL login screen. I also get a warning "Security Error, Domain Name Mismatch" stating the certificate may not belong to my domain and to contact a site administrator.
I'm not sure what is going on but this is a dramatic change. I've advised my users not to attempt to login to the website until this issue has been explained.
I checked under the "News" section of this forum and found nothing explaining the change.