KSEF-BOM
A check of the Polish KSeF FA(3) system – applies to invoices submitted to KSeF; a separate format, not Peppol.
What this error means
KSEF-BOM
The file starts with a UTF-8 BOM marker. KSeF requires UTF-8 without BOM.
How to fix it
Save the file as 'UTF-8 without BOM' (an encoding option in most editors) and upload it again.