Issues with pdf invoice after migration

-- HikaShop version -- : 6.1.0
-- Joomla version -- : 5.3.3
-- PHP version -- : 8.4.12

I finished migrating a webshop last week from joomla 3 to joomla 5. The webshop uses hikashop (ofcourse) and pdf attache invoice. Every week the owner of the shop sends the invoices. But since the migration the invoices are not working as expected. There is no invoice number created and the invoice is not send with the status that it is supposed to attach to.

At first I had the right invoice number but no attachment, so I updated the invoice plugin. That seemed to break the invoice numbers. I tried to delete the override made for the layout (flat layout) but that doesn't change anything.

I am out of options after over 2 hours of frustration about this. What am I missing here?