When I delete or ignore an alert on the essentials dashboard (Server 2016 Essentials with latest updates), it has a loading bar that never completes.
I found another related question at: https://server-essentials.com/community/answers/essentials-dashboard-not-ignoring-or-deleting-some-health-alerts
This does clear the alerts until one is generated again, but does not fix the problem where alerts cannot be deleted or ignored from the dashboard.
Probably related to .NET 4.7 or later getting installed I blogged this here:
https://www.mcbsys.com/blog/2018/06/essentials-2016-dashboard-fails-with-net-4-7-1/
Susan Bradley is also tracking here:
https://blogs.msmvps.com/bradley/2018/06/05/dashboard-issues-and-essentials/
Mark Berry