Snapshot Pro failure: has invalid filesize - should be minimum 18 bytes [code -10]

I am using Snapshot Pro for the first time, trying to back-up a WP multisite. It keeps failling though, with errors like this one:
ERROR: PcLZIP file:media add failed -10: gzip temporary file '/home/s88110/public_html/wp-content/uploads/snapshots/_backup/1461669001/pclzip-571f4d137d995.gz' has invalid filesize - should be minimum 18 bytes [code -10]
It first halted on the uploads/media/2013 sub folders, so I excluded them, but it keeps happening. The site contains viruses, which is why I desperately need a backup to restore to my test system.