Intermittent Cloud SQL, Export Failure - Code:2, Message:"UNKNOWN", severity:"ERROR" - google-apps-script

Question

When attempting to export from Cloud SQL using a RESTful call to the Google API we've been encountering random failures, approximately 1 in 10 attempts. The log extract below has been taken from Stackdriver and doesn't offer much insight into the root cause:
{
insertId: "55C4B416A7642.A1B5D8E.570939C6"
logName: "projects/my-dev-project/logs/cloudaudit.googleapis.com%2Fdata_access"
protoPayload: {
#type: "type.googleapis.com/google.cloud.audit.AuditLog"
authenticationInfo: {
principalEmail: "jsmith#mycompany.com"
}
authorizationInfo: [
0: {
authorizationLoggingOptions: {
permissionType: "DATA_READ"
}
granted: true
permission: "cloudsql.instances.export"
resource: "instances/my-db-instance"
}
]
methodName: "cloudsql.instances.export"
request: {
#type: "type.googleapis.com/cloudsql.admin.InstancesExportRequest"
exportContext: {
database: [
0: "business_db"
]
fileType: "CSV"
schemaOnly: true
selectQuery:
"SELECT
replace(IFNULL(`ID`,'<NULL_VALUE>'), '', '') AS `ID`,
replace(IFNULL(`NAME`,'<NULL_VALUE>'), '', '') AS `NAME`,
replace(IFNULL(`ADDRESS`,'<NULL_VALUE>'), '', '') AS `ADDRESS`,
replace(IFNULL(`DOB`,'<NULL_VALUE>'), '', '') AS `DOB`
FROM CUSTOMERS"
table: [
0: "CUSTOMERS"
]
uri: "gs://my-dev-project-stage/CUSTOMERS_full.csv"
}
instanceName: {
fullProjectId: "my-dev-project"
instanceId: "my-db-instance"
}
}
requestMetadata: {
callerIp: "107.178.192.158"
}
resourceName: "instances/my-db-instance"
serviceName: "cloudsql.googleapis.com"
status: {
code: 2
message: "UNKNOWN"
}
}
receiveTimestamp: "2017-10-24T13:52:53.738754270Z"
resource: {
labels: {
database_id: "my-dev-project:my-db-instance"
project_id: "my-dev-project"
region: "europe"
}
type: "cloudsql_database"
}
severity: "ERROR"
timestamp: "2017-10-24T13:52:53.317Z"
}
The only difference between this log entry and a successful export are the three property values mentioned in the post title. In a successful entry they are as follows:
status: {
}
}
receiveTimestamp: "2017-10-24T14:21:44.997050352Z"
resource: {
labels: {
database_id: "my-dev-project:my-db-instance"
project_id: "my-dev-project"
region: "europe"
}
type: "cloudsql_database"
}
severity: "INFO"
timestamp: "2017-10-24T14:21:43.757Z"
}
The intermittent nature suggests the root cause is an external factor. I.e. the exact same request with the same parameters; connection details, output file location and SQL statement will work a few minutes later. There's no consistency to which exports are failing.
We've already discovered that Cloud SQL does not support multiple simultaneous exports and we've implemented a queuing system to avoid this issue.
The data volumes don't seem to be an influencing factor either. An export will work for a 100,000 row file and then fail for 500 row file. In fact smaller or even empty data sets seem to be more susceptible to failure.
If anyone has any idea's or suggestions as to a solution or avenue of investigation I'd be interested to hear them. Thanks.