The error is because appdynamics adds a special singularity header on all soap calls to make correlation between appdynamics monitored application. Since, in this case of google adward service we don't need to correlete, you can create a custom backend rule from the controller UI to turn off this singularity header for this particular call.
Let me know if you need any help in doing the same.
Here is a document for reference - https://docs.appdynamics.com/display/PRO45/Backend+Detection+Rules
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