Snapshot is not backing up my files

I've been using Snapshot to backup my site and it's been working well backing up the files and folders up till about 3 months ago.

Now it just doesn't complete the backup and upload the zip file to Amazon S3.
I thought the problem is that I have an outdated plugin so I updated it to v2.4.3.4.
But am still having the same issue.

I checked the log file and I see lots of these errors:
Warning: errno:2 ZipArchive::addFile(): Invalid or unitialized Zip object /opt/bitnami/apache2/htdocs/partner/wp-content/plugins/snapshot/snapshot.php on line 3415

And at the end of the log, it doesn't show the usual log of transfering to AWS S3. It looks like it just stopped at one of the files it is backing up.

Another bit of info that maybe helpful is that the site with the issue is on Wordpress V3.8.5. I am not intending to update this wordpress for the moment. So want to see if there is something else in snapshot that is causing the error to happen.

Hope we can get to the bottom of this.