Synchronization error with Nextcloud birthdays

Well, it depends on which calendar has the issue. If it is a read-only calendar like a birthday calendar generated by the server, you can’t make changes. That means you can’t dismiss a reminder as that needs to be written back to the calendar and you don’t have permission to do that. Go to the Calendar section of eM Client and by right-click on that calendar folder choose Properties, and disable reminders.

Thank you for the tipp. So far it works, but I will see what happens when the next birthday comes around.