occasional AuthenticationError.CLIENT_CUSTOMER_ID_IS_REQUIRED error since Dec 14th

[David]

Hello community,
Since Dec 14th my python script (based o Python client library) started 
returning the error log below. I am truly puzzled by the behaviour of the 
error, as it pops up just occasionally and not consistently.

Initially I had no client_customer_id specified in googleads.yaml file, but 
after adding it there, error still persists.

Service called: GetService('AdwordsUserListService', 'v201609')

Error log:

<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/";>
   <soap:Header>
     
 <ns2:ResponseHeader 
xmlns="https://adwords.google.com/api/adwords/cm/v201609"; 
xmlns:ns2="https://adwords.google.com/api/adwords/rm/v201609";>
         <requestId>000544b8886eb8310a0dca0c460b8105</requestId>
         <serviceName>AdwordsUserListService</serviceName>
         <methodName>mutate</methodName>
         <operations>1</operations>
         <responseTime>24</responseTime>
      </ns2:ResponseHeader>
   </soap:Header>
   <soap:Body>
      <soap:Fault>
         <faultcode>soap:Server</faultcode>
        
 <faultstring>[AuthenticationError.CLIENT_CUSTOMER_ID_IS_REQUIRED @
 ; trigger:&apos;&lt;null&gt;&apos;]</faultstring>
         <detail>
           
 <ns2:ApiExceptionFault 
xmlns="https://adwords.google.com/api/adwords/cm/v201609"; 
xmlns:ns2="https://adwords.google.com/api/adwords/rm/v201609";>
              
 <message>[AuthenticationError.CLIENT_CUSTOMER_ID_IS_REQUIRED @ ; 
trigger:&apos;&lt;null&gt;&apos;]</message>
               <ApplicationException.Type>ApiException
</ApplicationException.Type>
               <errors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; 
xsi:type="AuthenticationError">
                  <fieldPath/>
                  <trigger>&lt;null&gt;</trigger>
                  <errorString>
AuthenticationError.CLIENT_CUSTOMER_ID_IS_REQUIRED</errorString>
                  <ApiError.Type>AuthenticationError</ApiError.Type>
                  <reason>CLIENT_CUSTOMER_ID_IS_REQUIRED</reason>
               </errors>
            </ns2:ApiExceptionFault>
         </detail>
      </soap:Fault>
   </soap:Body>
</soap:Envelope>

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