Hi @fsdg ,
The error message indicates that the calendar has more than 5000 items, which is the default threshold enforced by SharePoint Online . Unfortunately, it is not possible to increase this limit .
As a workaround, you can archive old items in the calendar to reduce the number of items within the threshold. One way to do this is to create a new calendar and move all old items to it that meet the following criteria: Older than a specific date / Non-recurring events and so on.
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