PDF/X-1a:2001 rendering is broken as of v18.4
Exporting using the PDF/X-1a:2001 preset no longer works properly as of versions 18.4. When rendering plate separations for printing using AFGA Apogee prepress software (which has NOT been updated, nor has the OS), the PDF crashes the render and spits out an error. I've checked and confirm that PDFs worked as expected forever until v18.4 and 18.5, which both produce an error. Saving the PDFs as a postscript within Acrobat and distilling back into PDF/X-1a:2001 fixes the problem, so something is broken in InDesign.
Apogee usually provides better error reports than the single "." in that screenshot, which suggests that it doesn't understand the nature of the problem.
Timo S. commented
Similar problem occurs over here: