It is documented in fiddler website.
Some workaround is
- Use machine name instead of localhost. That is http://my_machine_name/
- Use your local IP address. Check with ipconfig command
- If you're using Fiddler v2.1.8 or later, just use http://ipv4.fiddler to hit localhost on the IPv4 adapter, or use http://ipv6.fiddler to hit localhost on the IPv6 adapter
- Add a . at localhost. That is http://localhost./myweb