Error Backuping up using Snapshot

When making a snapshot and hitting Files: : themes

We get the following error: With an Abort....

ERROR: zip archive add failed [Array ( [type] => 8 [message] => A session had already been started - ignoring session_start() [file] => /home/cleanse2/public_html/wp-content/plugins/snapshot/snapshot.php [line] => 2156 ) ]

PLEASE ADVISE.