I believe this has been fixed with 9.0.8 9.0.8 | Tableau Software
This is caused by the dataengine being busy and therefore not responding timely to the polling for status. There is a workaround to change the polling frequency, % of success, etc. but 9.0.8 is the long term way to go and if you can live with the monitoring messages for now, then just do this.
Jeff is right. I wouldn't be too alarmed since the Data Engine comes back up within 15 seconds. See if you can upgrade to fix the issue.
Michel, did this answer it?