40215 is the reason code that we use when our database, MongoDB, returns an unexpected error to us.
You will want to check the "status_message" that is also returned for a more detailed description of the error being returned from the database.
The most common error message is "Failed to update record(s) for query: {<query details>}". Most of the time when this happens, it is because the JSON data that is being sent in the update contains a "$" or a "." - which are invalid in MongoDB keys.
Pro-tip:
You can use the brainCloud portal view the JSON data that was sent to the API method. Just look up the user in the User Browser, and go to the Summary > Logs page. Find the call that failed (look for a non-200 response), and then click on the eye in the Packet Id column to view the JSON data that was sent to the method. Search for "$" and "." to find the problem.
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