PDF/A files created with PDF Creator (no matter which format) are no longer valid.
To validate files I used http s ://bfo.com/blog/2017/11/08/verify_pdfa_online/
and http s ://telematici.agenziaentrate.gov.it/Documentale/ValidaDocumento.do?method=valida (the one more important for me).
there can be cases in which PDFCreator (freeware) creates PDF/A which don't validate in one or more PDF/A validators. PDFCreator Business uses a commercial library to create PDF/A, which is more reliable and should produce valid PDF/A for any input.
Or are you already using PDFCreator Business but still getting invalid PDF/A (if so, please send an example input document to support(at)pdfforge.org)?