The most common reason is that the Quarantine Digest has a URL with a private IP while they need a URL with a public IP. You'll need to verify a few settings:
- Under Config > Administration, make sure that Enable Outside HTTPS Administration is checked.
- Under Config > Administration > Public Address, choose Use Hostname or Use Manually Specified IP as appropriate.
- If using Use Hostname, make sure your hostname is properly configured and publicly resolvable at Config > Networking > Hostname. If you're using a Dynamic IP, it's recommended to set up Dynamic DNS on the same page.