Follwup ..... discovery ... after restoring a course that had legacy files successfully on a 3.7.highest sandbox, got an email from the 3.7 system on status of automated backups.
Error ... on the legacy course I restored without error ... missing was collapsed-topics. Hadn't installed that course format in the 3.7 sandbox.
There was an item in config of collapsed-topics for "legacy files'.
So ... by chance ... does your server have any course formats that might need updating?
IF not ... then I truely have come to the end of the road here!