I too struggled with this issue on a new "trail" hosted Confluence site, v1000.205.0. First you need to understand that the Calendar is an add-on, not an in-built feature. Then you need to know that it is added on your behalf and appears as a "user installed add on". These were points of confusion that made it difficult to know what "update the existing key" really meant. Here is what I did:
Are you an admin of the system? If so update the existing key - I assume the eval has expired...
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