Please ignore the message. In PDF/A-b generation scenarios (when all used fonts have to be embedded to the resulting PDF and a usage of built-in Acrobat’s Type1 fonts is not allowed), java:/defaultfonts points to a fallback font source. Usually the fonts are in pd4ml_rc.jar. If you’ve got all fonts in your /fonts folder, even PDF/A generation does not need java:/defaultfonts.

The invalid error message is to be suppressed in newer builds.