That particular, and very annoying, error has been around for a long time and a number of causes and solutions have been posited but I have never seen anything conclusive.
As to your speculation that changing your SSD from APFS to HFS+, how did you make the conversion without erasing/reformatting the SSD and re-installing High Sierra? I would think that should have taken care of any compatibility issues — unless perhaps the reinstall was cloned from another drive.
In either case your best bet may be to re-install MacOS from the Recovery Drive. When running a re-install from the Recovery Drive the
key combination you use to boot the Recovery Drive is critical. According to
Apple Support Article HT204904...
- Command+ R (⌘-R) Install the latest macOS that was installed on your Mac.
- Option+Command+R (⌥⌘R) Upgrade to the latest macOS compatible with your Mac.
- Shift+Option+Cpmmand+R (⇧⌥⌘R) Install the macOS that came with your Mac, or the closest version still available.
CAVEAT: I have not verified this for myself but I am taking Apple's word this will work. As with any OS change/upgrade/update
a known good backup/clone is HIGHLY recommended.
