1. Backup a course.
2. Restore that course, leaving all options default, until restore is complete and you are at the new site.
3. Now return to the original course and choose restore, again leaving all options default
'Error creating sections in the existing course.
An error has occurred and the restore could not be completed!'
This error occurs because the restore session was no longer cleared, probably as a result of changes to use required_param etc. I have checked in a fix and tested it works, but somebody who knows the code better might want to take a look. In http://moodle.cvs.sourceforge.net/moodle/moodle/backup/restore_check.html I changed:
if (isset($restore) and isset($restore_restoreto))
($restore_restoreto never was set) to
if (isset($restore) and isset($_POST['restore_restoreto']))