I went back to my original document, saved it for this conversion, in the Preflight window, chose PDF/X-1a (Coated FOGRA39).
After the conversion, at the top of the Prelifht Results tab, there is a green checkmark. It states: Preflight profile: "Convert to PDF/X-1a (Coated FOGRA39)" did not find any errors or warning. Lower down, it states, No problems found.
If I dig into the Overview of this results dialog, under Output intents, I found ICC output profile: Coated FOGRA39 (ISO 12647-2:2004)" and under that, Color space of data: "CMYK"
Further down, under Overview, there is Color spaces -> DevicGray color space.
Is this where the conflict lies?
Is it not possible to send you the original file for your inspection?
tmsrDD