This is likely due to a recent update to the holiday definitions, which may have caused the generated region list to change without updating users’ existing Calendar Region preferences.
The error occurs for users who previously selected one of the now-removed regions as their Calendar Region.
You’re seeing that many instances of the exception because the scheduled job that triggers this runs every 10 minutes.
We’ll need to handle this more gracefully, by accounting for holiday definition updates that remove regions from the generated region list.