I’m afraid I don’t remember exactly which validator that was. I tried a few such as veraPDF and Adobe Acrobat’s preflight verifier.

I found one of the preflight output files from Acrobat (but not the input document unfortunately) and these were the errors when validated against PDF/A-3a:

1. Device process color used but no PDF/A OutputIntent (101 matches on 1 page) - 1

2. Syntax problem: String object invalid (odd number of hexadecimal characters)

I think the second error was due to the BOM.