I don't have a solution either but try specifying a zip executable (Site Administration: Server/System Paths - "Path to zip") in the admin page instead of using PCLZIP.
If that doesn't work, then this could be a case not covered by the backup.What you should do then is start installing the latest Moodle 1.6 to a testing server, and restoring the course in there, followed by upgrading to 1.8. Backup the course from your testing 1.8 install and restore it to the server where you want it to go.
Backup and restore
Restore Error: PCLZIP_ERR_BAD_FORMAT (-10)
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