Moodle in English

We recently upgraded from 1.5.3 to 1.7 to fix some problems we were having from our previous upgrade. Everything is working fine again, except our backups don't work.

When we try and do a backup of a course, we get this error:

PCLZIP_ERR_MISSING_FILE (-4) : File '/moodledata/temp/backup/1164825320/course_files' does not exists

When I look in the moodledata folder, everything is there, but it doesn't create the zip file, which essentially makes it useless. Attached is a screenshot of the error. I don't think it's a file size thing, the course_files folder is 6.24MB, but we have a 8MB upload limit, but that shouldn't matter in this case.

Any ideas? thanks!