Google cloud healthcare export fails with error code 7

Google cloud healthcare export fails with error code 7. I'm exporting to GCS. Some objects are created in GCS. I see nothing appear, not even any kind of error, when I look in the related logs. The result I get is:

ERROR: (gcloud.healthcare.fhir-stores.export.gcs) {
  "code": 7,
  "message": "Operation execution failed."
}

When I look up the details with 'gcloud healthcare operations describe', I get this:

done: true
    error:
      code: 7
      message: Operation execution failed.
    metadata:
      '@type': type.googleapis.com/google.cloud.healthcare.v1.OperationMetadata
      apiMethodName: google.cloud.healthcare.v1.fhirstore.FhirStoreService.ExportResources
      counter:
        pending: '83263'

Am I missing some permissions somewhere?

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