Restored High Sierra 10.13.6 from Time Machine, now get: "Could not find APFS system volume handle"

I've been using MacOS 10.13.6 (build 17G4015) for a year now to retain use of my NVIDIA cards. Yesterday, I thought I should experiment with an upgrade to build 17G8030. Before doing so I did a full Time Machine backup. Once I realised I needed to return to my previous build, I booted in recovery mode and restored from Time Machine backup. Everything seemed to go OK until I tried to boot up and got the above error message. Does anyone have an idea to fix this? This SSD drive has been my OS drive for at least 2 years now and has been rock solid in High Sierra... why suddenly this APFS message. Thanks alot for any advice - Caspar