Still have the 1.9 site up and accessible?
Check /moodledata/temp/backup folder of the 1.9 server for files/folder contents. That might provide a clue.
Did the 1.9 backups include blocks/mod/ etc. addons that are not in the version 2.3 you are attempting to restore to?
On your 2.3 server ... do the same check of /moodledata/temp/backup for files/folders.
When version 2 of Moodle gets a .zip during a restore process it attempts to convert the backup.zip to a compatible .mbz backup first. All that process takes place in /moodledata/temp/backup/ Also, one might need 'legacy' at the course level and at the site level turned on. (Legacy, BTW, is not desired)
And there is a work-around that works most of the time ... but not always ...
Let's say you have a 1.9 backup for 'basket weaving' course (hereafter called 'bw') and you want to restore that to the 2.3.
In the 2.3, start a new course. Fill out minimum info and create a basket weaving course.
When you get to the enroll users screen of that course, go to the course menu 'restore'. Upload the bw.zip. Moodle will recognize it's not a .mbz and attempt to convert the bw.zip backup to a Moodle 2 format.
Eventually, one will get to a screen that displays options for where/how to restore. *** Important *** choose the option to restore to this course replacing the contents of this course ... the one you are currently in.
'spirit of sharing', Ken