This started happening to me a few days ago. I’m on 2.8.0.beta4 and I’ll upgrade to 2.8.0.beta6 later this week to see if that helps. I’m also using DigitalOcean spaces.
One interesting thing is that half my backups include the uploads and thumbnails and the other half do not. I was previously including the uploads and thumbnails but I decided to back those up separately using linux commands (i.e., rsync).
The fact that there are two backups on September 12 is not surprising - I created the second one manually after changing the settings to exclude the uploads and thumbnails. But there should not be two backups on September 13 or 14, and it should only be the backup without images.