@Tristan107 in this context Raspberry Pi Device Tree Overlays. They modify the behaviour of the system quite a bit, as you see it as well, so knowing what overlays are enabled makes a big difference, and helps us debug. So for example here the pi3-miniuart-bt was the relevant device tree overlay, which changes the availability of the serial port.
(Also, I think you don’t need the enable_uart setting anymore, just the pi3-miniuart-bt setting. I think…)