So, I tried around a little bit more and found out, that the "culprit" was the isTrigger checkbox in the capsule collider of the player prefab. Which is kind of understandable on one hand, but a little bit weird on the other hand. Because …
Thank you for the welcome and your answer :)
The PlayerStart was not created in the scene and therefore not assigned. I used the "Create" Button for the Default PlayerStart in the Scene manager to make a new one. Also I just tested it in …