During the Port Scan phase in ServiceNow Discovery, the warning level error "No results returned from probe" typically indicates a connectivity issue. This error often arises when a logical or physical firewall is blocking the connectivity between the MID Server and the target device. This prevents the probe from successfully scanning the ports and returning results. The other options, such as incorrect process classifier configuration, wrong credentials, or incorrect datasource precedence, would generally lead to different types of errors or issues in the discovery process. References = ServiceNow Discovery documentation, particularly the sections that deal with troubleshooting Port Scan issues and understanding common errors during Discovery phases.