Hi, welcome to the community.
I have seen this issue on my system before. It can happen on volumes which is have moved between filers and broken something in the DFM database. However the more common cause, is my DFM server is on a low spec server and complex reports beat the single DAS disk to death. The report times out and returns 0.
To test for this, use windows performance view and monitor local disk, ave queue length for the DFM database drive. If it is higher that 2 for a sustained period... Upgrade time
Hope it helps
Bren