Some time ago I reported on a solution for a problem with the PDF/X export in InDesign CC 2019.
With the update to InDesign 14.0.1 released on 19.11.18 (according to Adobe) the problem should also be solved.
Now, several readers have recently reported that the problem has reappeared in the latest version CC 2021 (version 16.1). (Unfortunately I can currently not reproduce the problem since I have to stick with an older MacOS version because of customer project.)
Two readers were able to bring back to live the export options for PDF/X-3 and PDF/X-4 by changing the color settings or reassigning a CMYK profile.
Please let us know in the comments (below) if you have also experienced this problem and if the suggested solution was able to fix the problem… Thanks!