In response to a series of questions concerning 'who' experiences the various stages of the Paticca Samuppada, The Buddha explains that these questions assume the idea of an individual or experiencer, and a differentiation between the experience and the experiencer and that such an assumption falls into the trap of postulating an eternal self or a self that is annihilated and that with either of those two extreme views it is not possible to end pain and reach the goal of Arahantship and that this amounts to blindness, but by bringing this blindness to an end and seeing that the process is impersonal the end of pain is attainable and the goal of Arahantship can be reached.
Read the Sutta
Index of Available translations: SN 2.12.35
This is a different set of questions than those which are referred to as 'not answered' by Gotama. Here he suggests the questions themselves are mis-phrased.
Be sure to check out the H.C. Warren translation!
Working back from the Buddha's response, we have a problem with all translators translating the first part of the bhikkhu's questions as "What is X?"
The question 'What is X?' is a valid one and is asked by The Buddha himself rhetorically in many of the previous suttas explaining the factors.
The PTS Pali and the CSCD puts this as a compound question rather than two separate questions; the BJT puts it as two separate questions.
As one question it can be heard as: 'What is, and what experiences X?'
And the Buddha's response then makes sense when he says that to pose the question that way implies an experiencer and a distinction between the experience and the experiencer and that where such distinctions are being made this implies one or the other of the speculative viewpoints called eternalism or annihilationism, both of which can be shown to be in error and which, if held, bar living the sort of life that would lead to the end of Pain and Arahantship.
See also the next sutta SN 2.12.36:
Almost identical to the previous but without the questioner. Mrs. Rhys Davids abridges completely. Here the BJT is in conformity with the PTS and CSCD in having the question posed as a single compound question. I suspect the first BJT was made to conform to the Rhys Davids translation.