I use Webmail to access my email account. There seemed to be an application "upgrade" for email and Calendar a few weeks ago.
Since then, I have been experiencing problems with Calendar:
When I delete one week's event out of a recurrent series, it appears to be removed (as before}. However, the next time I open the calendar, I find that the deleted occurrence has been reinstated. Very FRUSTRATING.
Also, every time I open the Calendar, there is a red banner about setting timezone. I have saved the correct (UTC) timezone on several occasions, but I still keep getting the banner.