Thank you for getting back to us.
We would like to update you that we were able to reproduce the error with Adobe Acrobat 9 Pro, but it did not reproduce with Adobe Acrobat 11 Preflight and Adobe Acrobat DC Preflight. Whereas, Adobe Acrobat Preflight tool reports only about one type of error - “Font is not valid”, this explanation is fuzzy and has no additional information about problems with font. All the fonts which were reported by Acrobat Preflight tool as “not valid” are CFF format fonts. We have checked the fonts as well and found that font structure and data is normal. So we are wondering what problem in font data calls an error for Acrobat Preflight tool.
Most probably this Preflight’s error is a bug of old Adobe Acrobat. Please note that these errors are reported by Acrobat 9 - this version of Acrobat is obsolete, when new versions of Acrobat - 11 and DC does not find any problem with converted PDF/A file.
We hope this information will be helpful. Please feel free to contact us if you need any further assistance.