This issue has been identified as K1-18451 After upgrading to 6.4, backups may not run for up to 7 days.
This issue causes the K1000 to continue different backup attempts after upgrade until the next scheduled full backup.
This issue is resolved in version 6.4 SP2.
Note: This situation has been addressed in a more recent version, please upgrade to the latest version. To obtain assistance with the appropriate upgrade path, see this article. For any versions necessary which are not listed in the technical documents/guides or downloads, please contact technical support.