This is a major show stopper.
Old groups were part of the course (courseid column in groups table). Our course backups are selfcontained - they handle only things INSIDE the course. Anything that is defined outside the course itself needs special UI to map those outside data - the first exception was roles.
The problem with groups and groupings is that they are not part of the course anymore - they live at the site level.
Current groups backup/restore code needs complete rewrite! It must handle both backup/restore on the same and different servers.