Tom, thank you for your response.
Following your questions:
1. Yes, I have this issue with the scenes from v.1.7 as I change my old projects all the time.
2. Yes, sometimes I leave unassigned light sources in the Rest channel as it's more convenient for me, and more importantly very often I use the Rest channel for the environment illumination in interior scenes (basically for skylight).
The resumed image appears without daylight from windows which ​makes the rendering in VFB much darker and unrealistic.
3. Sorry, the Rest channel is in the list of the resumed cxr file during the rendering process (I checked it one more time) however it looks like this channel is empty (does not contain any information), in other words when I click on the check box to show or hide it - nothing happens, the image stays dark without daylight from windows.
If I click on the same checkbox in the saved cxr file, then the Rest channel appears or disappears in VFB - in other words works normally.
If, as you mentioned the Rest (unassigned) channel/ layer has been changed in the newer versions then why it works normally in the saved cxr file, but does not work in the resumed?