When trying to backup my website (only files) I got an error "An unknown response returned from Snapshot backup attempt. Aborting. Double check Snapshot settings.".
Nothing special appears in the logs, it only stopped at section media/2017/02
I tried to exclude directory 2017/02 and the backup worked.
I think the issue is that directory 2017/02 size is 7.1Go and that snapshots don't manage to stats it, however I don't know what to change in settings to make the backup works (I changed max_execution_time to 1800 but without success)