On the DN and invoice there is a price printed at the product attributes, but the strange thing is that there are no attributes selected in that specific order.
Where do these prices come from?

Hello. Interesting; can you post here screenshot of invoice and also possibly screenshot of this order opened in VM invoice backend (with "attributes" opened)? Or you can send us your admin credentials to Tato emailová adresa je chráněna před spamboty, abyste ji viděli, povolte JavaScript and we can investigate this directly on site (please, include link to this topic).