Have you tried export them to html file?
That shopuld do the work. If bookmarks are all you need to be backed up/transferred to another computer.
Opera Sync often screw things up or does not work at all (in better case, hehe).
Anyway, you can tzry this:
Someone recommended it before and it did wonders for me after failing with all those manual backups from Opera tips and "how to's".
You can choose what to back up. Great little program.