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Last tested: Nov 5, 2018
This error is indicative of Looker trying and failing to start with new GCM encryption.
It means that this Looker is configured to use AWS Key Management Service (KMS) but KMS is unable to decrypt the stored key with the provided Customer Master Key (CMK).
Ensure that the database Looker is trying to start against is from this instance and hasn't been migrated from another host with a different CMK without doing DR backup procedure.