Snapshot Pro failing to backup

Snapshot Pro has been configured to backup to S3, it can see the bucket and was able to do a database backup without problem.

Trying to do a db+files backup fails every time. We have a lot of assets (15GB) so raised the memory limit to 1G and switched from pclZip to ZipArchive, but it still won't work.

Also, the snapshots folder is currently created under uploads, the plugin won't accept the new path outside the site root due to a permissions error when it tries to move snapshots to the new location. The destination folder is owned by nginx, outside the root with 777, but still fails.