While trying to export a .DXF file with 4.1.20 you may receive an error message "Board object missing user preferences". It appears there is an installation issue that occurs during the update. The fix for this is simple. Just navigate to your PCB123 installation directory and delete the Plugins folder, then reinstall PCB123.
Please note, this will not remove any custom libraries or your project files. This will simply have PCB123 reinstall the Plugins folder.