Notes and calendar have been linked, but the date is updated in the note, the calendar is not updated synchronously
Notes can be linked to an event, but this is only a uni-directional link: if you change the event, the date of the linked note is updated accordingly. In contrast, if you change the date of the note, you are essentially unlinking the note from the event. Instead you should edit the event to update the date of the note. You can edit the event directly from within Agenda by clicking/tapping it in the inspector on the right (premium function).
Most likely the calendar you tried to edit is not editable, best is to click on “Contact Us” and then send the email report as it contains the actual error that tells us more.