When moving my moodle-installation to a different server, I downloaded the moodledata-folder. Everything came down, except the files of the exercise-module. Only the filename was in the download, the actual files where empty.
I got a warning from my ftp-client (no access or file doesn't exist), so I used for that one course with exercises the build-in backupfunction.
filepermissions where 600 on the file, 777 on the parent folder(s). It's strange that it has a 600 permission isn't it?