There is a conflict with Snapshot and Constant Contact Forms. When Constant Contact is active, Snapshot's Managed Backups do not upload to WPMU DEV servers.
Once the Managed Backup reaches completion, this error appears on the Managed Backups screen:
<error empty response>
And the Managed Backups Log shows this:
[snapshot][2019-03-05 16:25:21][Error] Error spawning the S3 request handler, most probably due to a plugin conflict