The message "Canceling discovery because max run time window has been exceeded" is observed when trying to execute Discovery schedules.
The Business Rule 'Discovery - Complete' governs the Discovery process. Once the probings are successful, these should be marked as completed by this Business Rule, or otherwise, the process will be canceled. If the Business Rule has been altered from the OOB version and possibly not extensively tested, the administrator will need to investigate the changes and determine if the expected processing flow has been affected by missing relevant code.
Import the OOB version of the 'Discovery - Complete' Business Rule (shown below and attached) after saving the customization done for further development and testing.