I see the popup. looks to be a result of your popup blocker. but it gives you the option to allow the dialogs from the webmail. But a more permanent recommendation would be to add an exception to your popup blocker
I see, were you able to check out your browser's pop up settings as suggested above? "but a more permanent recommendation would be to add an exception to your popup blocker " if you did, please close and re-open your browser and test again.
I think if you click dismiss all and add them in the pop up settings they should stop, but since it is a calendar event it will likely happen again when there is a new occurrence to be honest.