objects with the AdGroupCriterionService, we are currently using v201601 of the API and have not made any changes on our side. Have there been any changes to this service?  See below for relevant SOAP request and response. 
<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope
  <SOAP-ENV:Header>
    <RequestHeader
      <clientCustomerId
      </clientCustomerId>
      <developerToken
      </developerToken>
      <userAgent
      </userAgent>
      <validateOnly
      </validateOnly>
      <partialFailure
      </partialFailure>
    </RequestHeader>
  </SOAP-ENV:Header>
  <SOAP-ENV:Body>
    <mutate
      <operations>
        <operator>ADD
        </operator>
        <operand
            xsi:type="BiddableAdGroupCriterion">
          <adGroupId>29068739164
          </adGroupId>
          <criterion
              xsi:type="CriterionUserList">
            <userListId>11461606624
            </userListId>
            <userListName>AudienceForSync31461606624
            </userListName>
            <userListMembershipStatus>OPEN
            </userListMembershipStatus>
          </criterion>
          <bidModifier>0.1
          </bidModifier>
        </operand>
      </operations>
      <operations>
        <operator>ADD
        </operator>
        <operand
            xsi:type="BiddableAdGroupCriterion">
          <adGroupId>29068739164
          </adGroupId>
          <criterion
              xsi:type="CriterionUserList">
            <userListId>21461606624
            </userListId>
            <userListName>AudienceForSync41461606624
            </userListName>
            <userListMembershipStatus>OPEN
            </userListMembershipStatus>
          </criterion>
          <bidModifier>10
          </bidModifier>
        </operand>
      </operations>
    </mutate>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>
<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope
  <soap:Header>
    <ResponseHeader
      <requestId>00053152cfa7db880ac11896a10d4cab
      </requestId>
      <serviceName>AdGroupCriterionService
      </serviceName>
      <methodName>mutate
      </methodName>
      <operations>2
      </operations>
      <responseTime>74
      </responseTime>
    </ResponseHeader>
  </soap:Header>
  <soap:Body>
    <soap:Fault>
      <faultcode>soap:Server
      </faultcode>
      <faultstring>[EntityNotFound.INVALID_ID @ operations[0].operand.criterion.userListId; trigger:11461606624, EntityNotFound.INVALID_ID @ operations[1].operand.criterion.userListId; trigger:21461606624]
      </faultstring>
      <detail>
        <ApiExceptionFault
          <message>[EntityNotFound.INVALID_ID @ operations[0].operand.criterion.userListId; trigger:11461606624, EntityNotFound.INVALID_ID @ operations[1].operand.criterion.userListId; trigger:21461606624]
          </message>
          <ApplicationException.Type>ApiException
          </ApplicationException.Type>
          <errors
              xsi:type="EntityNotFound">
            <fieldPath>operations[0].operand.criterion.userListId
            </fieldPath>
            <trigger>11461606624
            </trigger>
            <errorString>EntityNotFound.INVALID_ID
            </errorString>
            <ApiError.Type>EntityNotFound
            </ApiError.Type>
            <reason>INVALID_ID
            </reason>
          </errors>
          <errors
              xsi:type="EntityNotFound">
            <fieldPath>operations[1].operand.criterion.userListId
            </fieldPath>
            <trigger>21461606624
            </trigger>
            <errorString>EntityNotFound.INVALID_ID
            </errorString>
            <ApiError.Type>EntityNotFound
            </ApiError.Type>
            <reason>INVALID_ID
            </reason>
          </errors>
        </ApiExceptionFault>
      </detail>
    </soap:Fault>
  </soap:Body>
</soap:Envelope>
The error appears to be related to the CriterionUserList not *already* existing. Since this is an ADD operation I am confused as to why that fails, and why it only started failing recently.
Thanks.
Hunter.