Hmm it looks like this is related to
https://github.com/discourse/discourse/commit/83110a1a812140382167e8f5339c8bde3ee6610d
[2018-07-12 20:48:45] EXCEPTION: Failed to archive uploads.
tar: Option --warning=no-file-changed is not supported
Usage:
List: tar -tf <archive-filename>
Extract: tar -xf <archive-filename>
Create: tar -cf <archive-filename> [filenames...]
Help: tar --help
Can you try upgrading tar to at least 1.23?