On November 6th at 16:43:30 (CET) the Adobe preflight signature has expired. That’s why it is no longer possible to embed the preflight audit trail after a preflight with Acrobat Pro in a PDF:
After embedding the preflight results the PDF is usually “sealed” with this generic signature. This allows to recognize immediately if the PDF has been altered after the embedding of the preflight audit trail which may make the results obsolete. Thanks to the signature it is even possible to undo these changes.
Unfortunately the Adobe preflight signature is only valid for 5 years.
On November 3, 20 Adobe released just in time updates for Acrobat DC, 2020 and 2017 with a new preflight signature (valid until October 22, 2025).
Since there are unfortunately no more updates for older Acrobat versions from Adobe, you have to fix it yourself!