So after this and this post we went ahead and redid the whole loop with 90° fittings and improved layout and new tubes. The PC looks visually good as seen in the picture here. So we thought maybe our problems are solved now, a new and improved layout, a new mainboard, PSU, rad later this could be it. We thought it might run for the next year or so now without problem, but it was less then 15h until the pump broke down again. Honestly we all are at the edge a bit, we built this PC, fixed it again and again and redid the layout 2 times now. Can someone please tell us what cursed our loop and hardware?