What is up with this message?

What is up with this message?

 

I get this every couple of weeks. The NAS itself seems fine - SMARTS healthy and so on, still some free space on the partition, etc. It’s a ReadyNAS Duo, with upgraded 2TB drives. Starting a new backup seems to work, but I’ve been a bit too busy lately to test the backups, especially trying to restore something old from before creating the new backup. 

Am I okay, or do I now have placebo backups only?