What is wrong with my PDF file?

I need your help!

I have uploaded two PDF/A-1b files to the location:
https://share.acrobat.com/adc/document.do?docid=93b84f92-c603-42a9-992 5-3703805686aa

They are almost the same and created by our program.

c-5si.pdf is a PDF file without any problems after the inspection from Preflight Acrobat Professional V9.0 ( verify compliance withPDF/A-1b ).

However, c-ss.pdf got an error: CIDSet in subset font is incomplete.

What does this mean? Is there any document to explain this?

Thanks a lot!