This error is a SafeReindexingError, which is thrown when the difference between the number of records crawled between the two most recent crawls exceeds the value of the maxLostRecordsPercentage setting, which is set in your Crawler's editor.
It's designed to prevent accidental loss of records in the event of a misconfiguration.
You can remove this message by adjusting the maxLostRecordsPercentage setting within the safetyChecks parameter to a higher number.