In this screenshot, Cam4_ was the DCS-931L. The others are DCS-930L.

As with the DCS-930L and DCS-932L zero byte file issue reported some weeks ago, they tend to occur at even multiples of exact 20 minutes, e.g., 0800, 0820, 0840 -

I tested the 930 and 932 with and without time server sync, it did not matter.
This issue should be on the firmware wishlist for all of this series of cameras.