Got this error on cron archiving since update to Matomo version: 3.10.0-rc5:
Got invalid response from API request: ?module=API&method=API.get&idSite=8&period=day&date=last4&format=php&trigger=archivephp. Response was 'Matomo encountered an error: Cannot use object of type stdClass as array (which lead to: Cannot use object of type stdClass as array on /way/to/piwik/plugins/Tour/Tour.php(128) )'
Deactivate Tour plugin make archiving task complete with no error.
This server is still in php5, will be updated in next days.