Ubiquiti support told me that the "AP AC In Wall" only works in combination with one of the PoE switches (from Ubiquiti), because I couldn't get it to work with the separate injector. Maybe this has changed in the meantime (was 3 months ago), but it would explain why the setup from Morger85 works. (I had sent back my AP AC Wall" because I didn't have a PoE switch (yet) (the rest works via the "round" APs, e.g. Ubiquiti UniFi AP AC-PRO).

It is best to use the original Unifi. Or a network switch from Unifi, then there are no problems. I have connected everything via a Unifi switch and no longer need the Incetor. They are now in the drawer:-)