TimeMachineBackup must start clean version

I have made a TimeMachine backup of my iMac. The size of 550 GB took a couple of days of patience. But after a week the whole backup has gone: all space available again, but no backup.
What causes this? What is the solution?
This effectively makes the WD solution for compatible TimeMachine backups unusable. And RAID backups over WiFi was the main reason to by this NAS.

Hi there,

This is strange, I have not seen this happen before, the unit was not reset or any restore was made before this happened?