It took almost two hours, so i guess it was not "quick". I will try your link, thank you for your help. The fact i tried the same configurations on both cards make me think that the sandisk is failing. Fun fact that I bought it on holiday in Schöneweide at Mediamarkt, sadly a bit far away to return it. But ofcourse, it is not proven that the SD-card is damaged.
I will try different clustersizes aswell to maybe discover a pattern of the missing recordings. It stops at regular intervals and trying to store data on the card, as you mentioned. Maybe try to create a smaller partition on the card instead of allocating the entire space. I don't yet know what causes this problem