by Mary Cooch.
I think you CAN restore a course from 1.9 to 2.7 - but you cannot restore it with user data. If it doesn't have user data it should be possible to restore it, although I can't speak for older versions of 1.9.
Anyway - here are the search results for that error message: https://moodle.org/mod/forum/search.php?search=error%2Funknown_resource_successor&id=5
Worth reading through.