This warning is caused by references to users (in "jcr:lastModified" or "jcr:created" properties), who do no longer exist.
Please follow these steps to suppress the messages:
- Go to /system/console/slinglog
- Log level - ERROR
- Logger: com.adobe.granite.security.user.util.AuthorizableUtil
- Log file: error.log
Lastly, I see there has been a new backport request opened NPR-30977 for 6.5. So, the chances are this might get fixed in upcoming services packs.