WD Anwhere backup says destination is full but it isn't

Just started having this problem. My WD Anywhere Backup software is pausing because it says the destination is full. Yet if you look at the screen shot it also says there is 641 GB free. So what gives? backup destination is WD Worldbook NAS

Hello, unfortunately I have not used WD Anywhere backup, lets see if one of the more knowledgeable user is able to asssist.