I use Adobe X for simple booking forms and i save them as Reader Enabled to allow the user to save the completed form. Suddenly, I can't save any of the forms as Reader Enabled.
All worked fine before. Now none of them do, all give the same message "This document could not be Reader Enabled" but no clue why. I've tried reinstalling Adobe, and tried it on a different PC. Nothing seems to make any difference.
All old software is obsolete for reader extending. You need Acrobat DC subscription with the latest patch. Also, your end users need to use the latest Reader.
By the way, you do not need to Reader enable at all if you make forms in Acrobat, and the users use the latest Reader, and want to save! Reader enabling itself is obsolete (in this case).
The forms were created with Live Cycle Designer. I tried with the latest version of Acrobat reader but it is not possible to save it.
Yes, that's right. LiveCycle Designer forms (AEM forms, XFA forms) still need Reader extending. Either the latest Acrobat, or the latest server AEM version is needed.
Just to add to the correct suggestion by TSN above: This is happening because the certificate used for this right has expired. See: https://helpx.adobe.com/acrobat/kb/reader-extended-pdfs-and-adobe-root-ca-expiry.html