When using a Hat for the Raspberry (MotoPi from Joy-It) the onboard wifi doesn't connect to my home wifi. But when removing the hat it just works fine. The strange thing is, that I've used the same configuration without any changes in another network a week before.
Also, the distance shouldn't be a problem because I've tried right next to the hotspot. I've now made a new Raspbian install on the sd card, but the same problem occurs, when connecting via ethernet there's also no problem. It seems like the connection between the HAT and Raspberry causes the problem because when I hold the HAT just above the raspberry there's no problem.