I had this same error and found that my snapraid diff was definitely failing.
Try ssh into the box and going to /usr/sbin and running "sudo omv-snapraid-diff".
Mine failed with an error writing to a content file due to a read-only file system (my cause was a faulty SATA cable causing one of the content filesystems to be locked read-only - fixed by replacing the cable and fsck).
Another way to see if the diff is failing is checking "/var/log/snapraid.log". If you see 'Added:  - Deleted:  - Moved:  - Copied:  - Updated: ' instead of something like 'Added:  - Deleted:  - Moved:  - Copied:  - Updated: ', the diff is definitely failing so the script cannot parse the output correctly.
Hmm, seems running it manually works fine with no error and an email showing what was done.
Wonder why the cron job fails?