This is a guess as not run across the same issue ...
When attempting to restore a 1.9 course in a 2, MDL will attempt to re-construct the course into it's .mbz format.
It rebuilds in /moodledata/temp/backup/.
Failed restores have a hash named .log file. There is no Moodle 2 interface/button/link to those .log files so one has to view them via some other method. They are text files so anything that can view/open text files will do.
They might give you a clue. One that I've seen is 'no handler for' which is an indication that whatever that was, there is nothing compat in version 2 of Moodle. While it trys to skip those, solution to those would be to remove those blocks/activities in the 1.9 when creating the 1.9 backup and then try the restore again. In some cases, the 1.9 is restored successfully then. Notice I said 'some'! :|
The other item involved is the moodle.xml file at the root of the 1.9 backup. That also is text and could be inspected.
'spirit of sharing', Ken