The message from KDP says only
"I noticed that you have generated the mobi file using Calibre, and this is creating the problem. Please upload a mobi file generated using Kindlegen, and republish the title. It should then be available for sale."
No explanation of "the problem". When originally submitted, there as apparently no problem, for the book was made available for sale on the KDP website. I'll ask KDP for clarification.