Quite a few people started reporting having issues connecting to Cassini earlier in the week. I’ve seen blog posts, tweets and internal mail threads as people who had a working environment one day found the could no longer connect the next.
The solution is to re-add:
to your hosts file (%windir%\System32\drivers\etc). So your hosts file should at least contain the entries:
127.0.0.1 localhost ::1 localhost
It appears the problem may have been related to an update to the signature file for Windows Defender. The signature file was fixed the same day (the fix is in signature versions 1.53.283.0 and above). Make sure you update to the latest signature file asap.