In case anyone else encounters this, the problem is not triggered by an Oxygen update, but by a macOS update.
macOS 10.13 introduced a new font for emoji, "Apple Color Emoji.ttc" (~136MB ). This is huge by font standards.
The font detection that is used by both Apache FOP 2.2 and Oxygen PDF Chemistry 19.1 chokes on this particular font when trying to create the FOP fonts cache, <user_home>/.fop/fop-fonts.cache.
This issue affects all PDF transformations that use either Apache FOP or Oxygen PDF Chemistry.
A known workaround is to increase the memory for the FOP process to a very large value so that the detection passes at least once and creates the fonts cache.
e.g. For DITA transformations, edit the scenario, go to Advanced, JVM Arguments and specify -Xmx5000m (or -Xmx8000m).
e.g. For DocBook transformations (FOP only), go to Preferences, XML > XSLT-FO-XQuery > FO Processors, Memory available to the built-in FOP and specify 5000 (or 8000).
After running the PDF scenario successfully at least once (and the fonts cache was created), the memory can be reduced back to the default (or what value you have successfully used before).
We are currently working on a patch to avoid this problem.
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger