I had this issue around February this year. I had made no changes either. Very frustrating indeed.
My app finally got approved after I made a change to handle the "Deferred" payment state (SKPaymentTransactionStateDeferred) as a non-error.
Prior to this, anything other than SKPaymentTransactionStatePurchased was showing an "unable to complete in-app purchase" message.
Hope this helps.
willow1234
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