Hi,
I don't think this is linked to the update.
This error means that somewhere in the invoice you have a br tag not written properly.
It's hard to say where though.
What I would recommend is to add the code:
before the code:
$pdfContent[] = ob_get_clean();
in the file plugins/system/attachinvoice/attachinvoice.php and then try again.
That way, you'll get to see the HTML of the invoice before the PDF is generated out of it. Then, you can search for that br tag in the HTML of the invoice in order to better understand where the problem is coming from.
It could be the store address, it could be the name of a product, the address of the customer, the address format, etc.