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It seems that the plugin needs to account for the new spread operator introduced to WP Upgrader Skin in WP 5.3.
> PHP Warning Declaration of Ithemes_Sync_Upgrader_Skin::feedback($string) should be compatible with Bulk_Upgrader_Skin::feedback($string, …$args) in /sites/thefragens.net/files/wp-content/plugins/ithemes-sync/upgrader-skin.php on line 0