Hi, Selene.
The original issue where Custom Field Ids are causing reports to fail is still being looked into by our engineers. I just checked, and I can confirm that the original request is failing with customFieldIds, but it succeeds when customFieldIds are removed. This request has several AD_EXCHANGE_ metrics and succeeds, so I think your issue is unrelated.
Please open a new topic in the DFP API Forum documenting the behavior you are seeing. Also, including your SOAP logs will help us get to the cause of your issue.
Thanks,