Hi,
Thanks for the log access.
I was able to get the error message from it:
Backend fatal error: PHP Fatal error: Array and string offset access syntax with curly braces is no longer supported in /home/deb112713/domains/schrijverspunt.nl/public_html/plugins/hikashop/attachinvoice/vendor/tecnickcom/tcpdf/tcpdf.php on line 16900
So you do have the PDF invoice plugin installed, and it is activated, and it is an old version which isn't compatible with PHP8 :
i.imgur.com/WWmU19O.png
Either disable it, or update it and the invoice will work again.