while there is no way to set this directly, this is caused by Ghostscript 9.16 which is only present in PDFCreator 2.1.2.
Downgrading to PDFCreator 2.1.1 should make it produce PDF 1.4 in all cases where it is possible (higher PDF level will be required to use AES high level encryption).
while PDF/A2-b supports features of PDF v1.5 (even 1.7 according to wikipedia, not sure if that is correct though), it doesn’t require them to be used and PDFCreator will always create the lowest possible PDF (>=1.4) version according to the files content.